If you’re having a problem updating Windows Defender’s definitions try rolling back the definitions to the last set or completely uninstall all signature definitions and engine files.

Roll Back the Signature Definitions

1. Click the Start orb, All Programs, Accessories, and then right-click Command Prompt and select Run as Administrator to start an elevated command prompt.
2. Type C:\Program Files\Windows Defender\MpCmdRun.exe –RemoveDefinitions in the command prompt.
3. Press Enter on your keyboard.

Windows Defender’s signature definitions will be rolled back to last set. You can try re-downloading the latest signature definitions.

Uninstall All Signature Definitions and Engine Updates

1. Click the Start orb, All Programs, Accessories, and then right-click Command Prompt and select Run as Administrator to start an elevated command prompt.
2. Type C:\Program Files\Windows Defender\MpCmdRun.exe –RemoveDefinitions –All in the command prompt.
3. Press Enter on your keyboard.

Windows Defender’s default signature definitions and engine will be loaded. You can re-download new signature definitions and engine updates now.