My computer has been restarting randomly

KERNEL_SECURITY_CHECK_FAILURE (0x139) is a stop code error, which means Windows detected a condition that compromised safe system operation and restarted to prevent corruption.

For random restarts on Windows 11, use this order:

1. Remove any recently added hardware or peripherals Since the issue is intermittent, disconnect any newly added external devices and test. Unexpected restarts can be caused by hardware devices or their drivers.
2. Start in Safe Mode If restarts continue, start the PC in Safe Mode and test there. This helps determine whether the cause is a driver or software loaded during normal startup.
3. Check Device Manager for driver problems
   • Right-click Start > Device Manager
   • Look for any device with an exclamation mark !
   • Right-click that device > Update driver
   • If updating does not help, try Disable device or Uninstall device
4. Check free disk space Keep 10% to 15% free space available on the drive. Windows and apps need free space for swap files and other operations.
5. Install Windows updates again after testing Pausing updates can help isolate the issue temporarily, but Windows should be brought back up to date.
   • Go to Start > Settings > Windows Update > Check for updates
6. Repair Windows system files If the problem started around updates or system instability, run these from Command Prompt as administrator:
   • DISM.exe /Online /Cleanup-image /Restorehealth
   • then sfc /scannow
   Expected result: Command Prompt should report that the restore operation completed successfully, and the SFC scan should reach 100%.
7. If Windows Update itself is failing, reset the update components Warning: these are administrator command-line steps. Run these commands one at a time in Command Prompt as administrator:
   • net stop bits
   • net stop wuauserv
   • ren %systemroot%\softwaredistribution softwaredistribution.bak
   • ren %systemroot%\system32\catroot2 catroot2.bak
   • net start bits
   • net start wuauserv
   Then restart the PC.
8. Check the hard drive for errors In Command Prompt as administrator, run:
   • chkdsk /f C:
   Then restart when prompted.
9. Use recovery options if the issue continues If none of the above helps, restore Windows using a restore point or another recovery option.
10. If the PC fails to start reliably Restart the PC first. On Windows 11 version 24H2 or later, Quick Machine Recovery can automatically detect repeated boot failures and try to apply a fix from Windows Update if enabled.

The Secure Boot event in Event Viewer is not enough by itself to identify the cause from the available information. The supported next step is to follow the stop-code troubleshooting path above, especially Safe Mode, Device Manager, system file repair, disk check, and then Windows Update repair if updates are involved.

References:

• Troubleshooting Windows unexpected restarts and stop code errors
• Troubleshoot problems updating Windows
• Recovery options in Windows
• Bug checks (stop code errors)