| Error Message | Likely Cause | 64-bit Specific Fix | | :--- | :--- | :--- | | | Third-party AV active | Uninstall the other AV, reboot. | | "Microsoft Defender Antivirus is turned off" | Disabled via GP/Registry | Use gpedit.msc or regedit as shown above. | | "Virus & threat protection unavailable" | Corrupted Windows Security app | Run: Get-AppxPackage Microsoft.SecHealthUI | Reset-AppxPackage in PowerShell (Admin). | | "No available antivirus provider" | Missing Defender services | Press Win + R , services.msc . Find Microsoft Defender Antivirus . Set Startup to Automatic. Start it. | | Definition update fails with 0x80070643 | 64-bit driver conflict | Run the manual mpam-fe.exe downloaded from Microsoft. |
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