Error codes

0x800106ba

Windows Defender service failed to start

What it means

The Microsoft Defender Antivirus service could not start, so Windows Security shows an error instead of protection status.

Common causes

• The WinDefend service is disabled
• Another antivirus took over and left Defender off
• Malware or a tweak tool changed the service

How to fix it

1. In services.msc, set Microsoft Defender Antivirus Service to Automatic and start it.
2. If a third-party antivirus is installed, either keep it and accept Defender staying passive, or uninstall it to let Defender take over.
3. Restart and check Windows Security again.
4. If it still fails, run sfc /scannow and a Microsoft Defender Offline scan.