Start with this diagnostic checklist
Before replacing anything, answer these questions:
- Which program shows the error, and does it appear during startup, login, or when networking begins?
- Is the file located in an old security-software folder, or in
System32,SysWOW64, Downloads, or another writable folder? - Is the affected application 32-bit or 64-bit?
- Did the message appear after uninstalling, updating, quarantining, or cleaning up an old security product?
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Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →filterlsp.dll is not a Windows system file. The strongest historical association is with PC Tools Spyware Doctor, where it acted as part of a Layered Service Provider, or LSP, network filter. Public file databases describe it as “Cair LSP Filter,” but that label is not proof that every copy came from the same product.
The safest repair is to identify the application that installed the file, then repair or remove that application. Do not begin by downloading a replacement DLL.
What filterlsp.dll does
An LSP can sit inside the Windows Winsock networking chain used by applications to connect to the internet. A security product may use this type of component to inspect or filter network traffic.
Historically, filterlsp.dll has been associated with PC Tools Spyware Doctor and a path similar to:
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C:Program FilesSpyware Doctorfilterlsp.dll
The exact parent application cannot be known from the filename alone. A copy in an old Spyware Doctor directory is materially different from a copy in C:WindowsSystem32, C:WindowsSysWOW64, a temporary folder, or a user-writable location.
PC Tools security products are legacy software. Symantec withdrew the PC Tools security portfolio in 2013, and a current official installer or supported repair package may no longer be available. If you still have the original installer, use it. If not, removing the obsolete product and repairing Winsock is safer than obtaining a DLL from an unofficial source.
Fix 1: Identify the owning application and file
Find the file path
If the error window names a path, record it. Otherwise search for the file:
- Open File Explorer.
- Select This PC.
- In the search box, enter
filterlsp.dll. - Wait for the search to finish.
- Right-click each result and choose Open file location.
Check the folder name and surrounding files. A path under an old PC Tools or Spyware Doctor directory supports the historical association. A copy in a system directory does not automatically make it legitimate.
You can also inspect a specific file with PowerShell. Replace the example path with the actual location:
Get-Item "C:pathtofilterlsp.dll" |
Format-List FullName,Length,LastWriteTime,VersionInfo
Get-AuthenticodeSignature "C:pathtofilterlsp.dll"
Get-FileHash "C:pathtofilterlsp.dll" -Algorithm SHA256
Look for a valid signature, a plausible installation directory, and a security result that does not identify the file as malicious. Public databases list historical metadata for this filename, but those records are not authoritative release documentation.
Check whether the parent program remains installed
- Open Settings.
- Select Apps.
- Open Installed apps.
- Search for Spyware Doctor, PC Tools, or another old security or network-filter product.
- Select the three-dot menu beside the program.
- Choose Advanced options if available.
- Use Repair if Windows offers it.
On older Windows installations:
- Open Control Panel.
- Select Programs.
- Open Programs and Features.
- Select the suspected product.
- Choose Repair or Change, if available.
Restart Windows and test the program that produced the error. If it starts without the message and networking still works, the repair succeeded.
Do not delete filterlsp.dll before repairing or uninstalling its parent application. An LSP can also be referenced by the Winsock catalog, so removing the file first can leave a broken network configuration.
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If the owning application is installed, use its original installer or its Windows uninstall entry.
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Repair with the original installer
- Locate the original installer from your own software records or trusted installation media.
- Right-click it and select Run as administrator.
- Choose Repair if offered.
- Complete the installation.
- Restart Windows.
- Open the application that reported the error.
If the error disappears, leave the DLL in the application’s installation folder. Do not copy it into System32 or SysWOW64.
Uninstall the obsolete product
If repair is unavailable or the product is no longer wanted:
- Save any needed settings.
- Close running applications.
- Open Settings → Apps → Installed apps.
- Find the old PC Tools or Spyware Doctor entry.
- Select the three-dot menu and choose Uninstall.
- Follow the uninstaller.
- Restart Windows.
- Test internet access and the application that originally showed the error.
If the uninstaller fails, do not repeatedly delete program files by hand. The network filter may still be registered even after part of the application has been removed. Continue with the Winsock procedure below.
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If the original installer is unavailable, do not search for an unauthenticated “latest” PC Tools package. There is no verified current replacement installer to recommend for this legacy product.
Fix 3: Reset the Winsock catalog
This is the most relevant repair when the error began after uninstalling Spyware Doctor or another old network-filter product, especially if internet access is also broken.
First record the current catalog:
- Open Start.
- Type
Command Prompt. - Right-click Command Prompt.
- Select Run as administrator.
- Approve the User Account Control prompt.
- Run:
netsh winsock show catalog
Look for entries associated with the removed security product or an old filter. Then run:
netsh winsock reset
Restart Windows immediately afterward.
The reset rebuilds the Winsock catalog. It does not reinstall filterlsp.dll. After the restart, check:
- Whether Windows reaches the internet
- Whether browsers and other network applications connect
- Whether the original startup message is gone
- Whether VPN, antivirus, proxy, or parental-control software still works
Winsock reset can remove registrations used by other legitimate network-filter products. If a VPN or endpoint-security program stops working, repair or reinstall that product from its official installer.
If networking remains broken, open Command Prompt as administrator again and run:
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ipconfig /flushdns
ipconfig /release
ipconfig /renew
These commands refresh DNS and the network lease, but they do not replace the DLL. If the problem is specifically a stale LSP registration, the Winsock reset is the important step.
Fix 4: Check antivirus quarantine before restoring anything
A missing-file message can appear because security software quarantined or deleted the original file. That does not prove the file was safe.
In Windows Security:
- Open Start.
- Type Windows Security and open it.
- Select Virus & threat protection.
- Choose Protection history.
- Review entries related to
filterlsp.dll, Spyware Doctor, or the parent application. - Open an entry to see its detected path and action.
Restore the file only when all of these are true:
- The original path belongs to trusted installed software.
- The file’s publisher and signature are acceptable.
- Your security software does not identify it as malware.
- You intend to keep and repair the parent application.
If the file came from a strange writable directory, has no credible installation context, or is detected as malicious, do not restore it. Remove the obsolete application and run a full security scan instead.
If quarantine removed a legitimate file but the parent program is still needed, reinstall or repair that program from its original official installer rather than restoring an isolated DLL.
Fix 5: Check Windows components, but do not expect SFC to restore this file
filterlsp.dll is identified as a third-party application file, so Windows system repair is not the primary solution. Use DISM and SFC when you also have broader Windows symptoms, such as damaged built-in tools, update failures, or multiple protected-file errors.
- Open Command Prompt as administrator.
- Run these commands in order:
DISM /Online /Cleanup-Image /RestoreHealth
sfc /scannow
- Wait for each command to finish.
- Restart Windows.
- Test the affected program again.
DISM repairs the Windows component store. SFC checks protected Windows system files. Neither command is a dependable way to restore a third-party filterlsp.dll.
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If SFC reports no integrity violations, that does not disprove a missing filterlsp.dll. It usually means protected Windows files are intact.
Fix 6: Update or reinstall related network drivers
A driver update is not the first repair for this DLL, but it is reasonable if the error is accompanied by a failed network adapter, dropped connections, or a recent driver change.
- Right-click Start.
- Select Device Manager.
- Expand Network adapters.
- Right-click the affected adapter.
- Select Update driver.
- Choose Search automatically for drivers.
- Restart Windows if prompted.
If the adapter stopped working after an update:
- Return to Device Manager → Network adapters.
- Right-click the adapter and choose Properties.
- Open the Driver tab.
- Select Roll Back Driver if available.
For a damaged installation, choose Uninstall device, select the option to remove the driver only if Windows clearly offers it and you have a replacement source, then restart and install the driver from the computer or adapter manufacturer.
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Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why regsvr32 usually does not help
Do not treat this as a general DLL-registration problem. The command:
regsvr32 filterlsp.dll
is intended for DLLs that provide the registration entry points expected by regsvr32. An LSP is normally registered through Winsock configuration, not by manually registering the file.
Running the command may produce an error such as a missing entry point or may appear to do nothing useful. It does not rebuild a damaged application, restore a Winsock catalog entry, or supply missing dependencies.
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Do not copy a replacement into:
C:WindowsSystem32C:WindowsSysWOW64- The Windows directory
- An unrelated application folder
A 32-bit application generally uses files from its own installation directory or, on 64-bit Windows, commonly from locations associated with 32-bit applications. A 64-bit application requires a compatible 64-bit component. Mixing architectures can produce “not designed to run on Windows” errors and may create a second problem.
Fix 7: Use System Restore only when the timing is clear
If the error began immediately after a failed uninstall or update, System Restore may undo the related system configuration.
- Open Start.
- Search for Create a restore point.
- Open it.
- Select System Restore.
- Choose a restore point created before the failed change.
- Review affected programs.
- Start the restore and restart Windows.
System Restore is conditional, not a substitute for repairing the owning application. Create a current restore point before making further changes when possible.
Avoid random DLL downloads
Downloading filterlsp.dll from an unknown DLL site is risky because the file may contain malware, have the wrong architecture, belong to a different release, or fail to match the application’s dependencies. It also leaves the original Winsock registration or damaged installation unresolved.
Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsUse these sources instead:
- The original installer for the application that owned the file
- The application developer’s official support or download channel, when one still exists
- Windows Settings and Microsoft’s built-in repair commands for Windows components
- The computer or network-adapter manufacturer for drivers
If no trustworthy installer remains for the legacy PC Tools product, uninstalling it and resetting Winsock is safer than introducing a standalone DLL.
When the error still appears
If the popup remains after uninstall and Winsock reset:
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- Restart Windows once more.
- Check Settings → Apps → Installed apps for a remaining related security product.
- Review Task Manager → Startup apps for leftover entries.
- Search the application’s installation folder for an uninstaller.
- Check Event Viewer → Windows Logs → Application for the program name or failing path.
- Confirm that the popup identifies the same
filterlsp.dlllocation. - Run a security scan if the file is in an unusual directory.
If networking is still broken, repair VPN, antivirus, proxy, or parental-control software after the Winsock reset. If only one application fails, reinstall that application rather than repairing Windows globally.
FAQ
Is filterlsp.dll a Windows file?
No. It is a third-party application DLL and should not normally be installed in System32 or SysWOW64.
What program installed filterlsp.dll?
The best-supported historical association is PC Tools Spyware Doctor. The filename alone cannot prove that every copy came from that product.
Is filterlsp.dll safe?
The filename alone proves nothing. Verify the path, digital signature, hash, and antivirus result. A copy in the historical application directory is less suspicious than one in an unusual writable folder, but it still needs verification.
Does Windows Update replace it?
No. Windows Update is not a reliable source for a third-party application DLL.
Should I run SFC for this error?
Run DISM followed by SFC only when there are wider Windows integrity symptoms. These commands are not the primary repair for filterlsp.dll.
What is the safest fix after uninstalling Spyware Doctor?
Run netsh winsock reset from an elevated Command Prompt, restart Windows, and repair any VPN or security software affected by the catalog reset.
Why does “not designed to run on Windows” appear?
Possible causes include a damaged file, wrong architecture, missing dependencies, malware, or a failed installation. It does not automatically mean Windows itself is corrupt.
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