how to fix windows defender

Try this in order:

1. Restart the PC Close apps and restart Windows first.
2. Check the Defender service
   • Open Start and run services.msc
   • Find Windows Defender Antivirus Service
   • Make sure Startup type is Automatic
   • On the Action menu, select Start
   • If Start is unavailable, select Stop, wait for it to stop, then select Start again
3. Check whether another antivirus is disabling Defender On Windows 10/11, if a non-Microsoft antivirus is installed, Microsoft Defender Antivirus can be turned off automatically.
   • Open appwiz.cpl
   • Uninstall any third-party security or antivirus software
   • Restart the PC
   Note: some security apps do not uninstall completely and may require their cleanup utility. Removing security software can leave the PC temporarily unprotected.
4. Review Defender event logs
   • Open Event Viewer
   • Go to Applications and Services Logs > Microsoft > Windows > Windows Defender > Operational
   • Check for these event IDs:
     • 15
     • 5007
     • 5010
5. Check if policy is turning Defender off Open Command Prompt as administrator and run:

      GPresult.exe /h gpresult.html
   

   Open the generated gpresult.html report and check under Windows Components/Microsoft Defender Antivirus for:
   • Policy: Turn off Microsoft Defender Antivirus
   • Setting: Enabled
   Also check whether DisableAntiSpyware is being applied under: HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender
6. Check Defender-related services in PowerShell Open PowerShell as administrator and run:

      Get-Service WinDefend, WdBoot, WdFilter, WdNisSvc, WdNisDrv, SecurityHealthService, wscsvc | Format-Table -Auto DisplayName, Name, StartType, Status
   

   Expected key results include:
   • Microsoft Defender Antivirus Service (WinDefend): Automatic, Running
   • Windows Security Service (SecurityHealthService): Manual, Running
   • Security Center (wscsvc): Automatic, Running
7. Reset Defender definitions and platform Open Command Prompt as administrator and run:

      MpCmdRun.exe -RemoveDefinitions -All
      MpCmdRun.exe -ResetPlatform
   

8. Back up and remove Defender policy keys This changes Defender policy in the registry. Back up first. Open PowerShell as administrator and run:

      New-Item -Path "C:\DefenderTemp" -ItemType Directory; Invoke-Command {reg export 'HKLM\SOFTWARE\Policies\Microsoft\Windows Defender' C:\DefenderTemp\_DefenderAVBackup.reg}
   

   Then remove the Defender policy key:

      Remove-Item -Path 'HKLM:\SOFTWARE\Policies\Microsoft\Windows Defender' -Force
   

9. Run Windows Update After the steps above, run Microsoft Update/Windows Update and install available updates.
10. If the service still will not start Run the Microsoft Safety Scanner to rule out malware. If errors appear when starting the service, contact Microsoft Support and provide the exact error.

References:

• Troubleshoot Windows Defender or Endpoint Protection client
• Troubleshoot Microsoft Defender Antivirus while migrating from a non-Microsoft solution
• Troubleshoot Microsoft Defender Antivirus service startup problems