Your PC couldn't be scanned

Install all Windows and Defender protection updates, restart the PC, and retry the scan before using Microsoft Defender Offline if the normal scan still fails.

1. Update Windows and Defender

1. Open Start → Settings → Update & Security → Windows Update.
2. Select Check for updates, install everything offered, and restart Windows.
3. Open Windows Security → Virus & threat protection.
4. Under Virus & threat protection updates, select Check for updates, then check again for updated threat definitions.

Microsoft recommends both Windows Update and current protection updates when Defender repeatedly encounters scan errors.^1^

2. Retry under clean conditions

1. Ensure drive C: has adequate free space; Defender needs space to quarantine and remove threats.
2. Restart the PC.
3. Before opening other applications, go to Windows Security → Virus & threat protection → Scan options.
4. Run Quick scan first. If it completes, run Full scan.

Closing other applications reduces competition for processor and memory resources during scanning.^1^

3. If normal scans still fail, run an offline scan

Save all open work first because this scan restarts the computer.

1. Open Windows Security → Virus & threat protection → Scan options.
2. Select Microsoft Defender Antivirus (offline scan).
3. Select Scan now.
4. Allow the computer to restart and complete the scan.
5. After Windows restarts, open Windows Security → Virus & threat protection → Protection history to review the result.

The offline scan runs in the Windows Recovery Environment, where persistent malware has more difficulty hiding or interfering with Defender.^2^

Conditional checks

• If another antivirus is installed: A compatible third-party antivirus normally causes Microsoft Defender Antivirus to turn itself off.^2^ Use that product for scanning, or uninstall it through Settings → Apps → Apps & features, restart, and then retry Defender. Do not use machine-specific registry scripts or “fix lists” from another case.
• If the error continues after updates and the offline scan: Press Windows key + F, describe the exact scan type and error in Feedback Hub, and submit it under Security, Privacy, and Accounts → Windows Defender Antivirus. Microsoft explicitly requests feedback for recurring scan errors.^1^
• If malware is known to have disabled or damaged Defender: Back up important files and use Windows recovery or repair options. Reinstallation or reset is a last resort for irreversible malware changes, not the first response to this message alone.^1^

References

1. Troubleshoot problems with detecting and removing malware | Microsoft Support
2. Virus and Threat Protection in the Windows Security App