Windows Error 0x80073B01 — Windows Defender Service Failed to Start
Errorsecurity
Overview
Windows error 0x80073B01 means the Windows Defender antimalware service failed to initialize, leaving your system without real-time virus protection.
Key Details
- Error 0x80073B01 prevents Windows Defender from starting its real-time protection service
- Often caused by remnants of third-party antivirus software that was not fully uninstalled
- Registry entries from old AV software can block Defender from initializing
- Windows Security app may show 'Your virus & threat protection is managed by your organization'
- Group Policy settings can also prevent Defender from starting
Common Causes
- Remnants of third-party antivirus (Norton, McAfee, Avast) blocking Defender
- Corrupted Windows Defender program files or service registration
- Group Policy or registry settings disabling Defender
- WinDefend service set to Disabled in Services
- Missing Windows Defender platform update
Steps
- 1Use the AV vendor's official removal tool to completely clean up old antivirus remnants
- 2Check services: services.msc > Windows Defender Antivirus Service > set to Automatic and Start
- 3Check registry: HKLM\SOFTWARE\Policies\Microsoft\Windows Defender > delete DisableAntiSpyware if present
- 4Run SFC /scannow and DISM /Online /Cleanup-Image /RestoreHealth to repair system files
- 5Reset Defender: PowerShell (admin) > Get-AppxPackage Microsoft.SecHealthUI -AllUsers | Reset-AppxPackage
Tags
windowsdefender0x80073b01antivirusservice
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WarningFrequently Asked Questions
Third-party AV registers as the system's security provider. If not fully removed, Windows thinks another AV is installed and keeps Defender disabled.