My Antivirus Microsoft Defender has stopped working

To use Microsoft Defender Antivirus for scans, uninstall the currently installed third-party antivirus and restart Windows 11; Defender is normally disabled automatically while another antivirus is active and turns back on after that product is removed.^1^^2^

Remove the other antivirus

Before uninstalling, ensure that you have its product key or account details if you may reinstall it later.

1. Open Start > Settings > Apps > Installed apps.
2. Find the third-party antivirus.
3. Select its … menu, then Uninstall.
4. Complete the vendor’s removal wizard.
5. Select Start > Power > Restart. Use Restart, not Shut down.
6. Open Windows Security > Virus & threat protection.
7. Under Virus & threat protection settings, select Manage settings and check that Real-time protection is on.
8. Return to Virus & threat protection, select Quick scan, and confirm that the scan begins.

Microsoft recommends not running multiple real-time antivirus products simultaneously because this can cause errors and performance problems.^3^

If you intend to keep the third-party antivirus, use that product’s scan function instead. Defender not starting an on-demand scan is expected when the non-Microsoft product is registered as the primary antivirus.

If Defender does not return after removal

An incomplete uninstall may leave the former antivirus registered or retain components that block Defender.

1. Check Settings > Apps > Installed apps and confirm that all components belonging to the antivirus vendor have been removed.
2. If removal failed, select the antivirus under Installed apps > Advanced options and use Repair or Reset if those options are available; otherwise, obtain the vendor’s official removal utility or contact its support.^4^
3. Restart Windows again.
4. Go to Start > Settings > Windows Update > Check for updates and install all available updates.
5. Reopen Windows Security and retry Quick scan. Microsoft recommends Windows Update when Defender repeatedly encounters scan errors.^5^

Do not manually change Defender registry keys or force-start its service while another antivirus remains installed; Windows is intentionally managing Defender’s state for compatibility.

References

1. Microsoft Defender Antivirus and non-Microsoft antivirus solutions without Defender for Endpoint
2. Windows Security App Overview | Microsoft Support
3. Antivirus and antimalware software: FAQ | Microsoft Support
4. [Windows 11](https://support.microsoft.com/windows/deployment/install-upgrade/fix-problems-that-block-programs-from-being-installed-or-removed#summary)
5. Troubleshoot problems with detecting and removing malware | Microsoft Support