Error Code 0xC000021A is a driver fault that can can happen when driver mismatches exist. This usually arises when driver uninstallers fall to remove all obsolete components when upgraded. This error has been seen with Windows XP onwards.

The error is thrown when Winlogon.exe or Csrss.exe fail. The error arises when the driver stack is not consistent.

Sometimes the Sytem File Checker can fix this problem.

Open an elevated command prompt and run:

sfc /scannow

It may take some time.

You may need to use Safe Mode to remove drivers. If that does not work, you may be forced to backup your files and install Windows clean.