The Stop 0xC000021A error message occurs when Windows Vista switches into kernel mode and a user-mode subsystem is compromised and security can no longer be guaranteed.
A Stop 0xC000021A error message can also occur after a system is restarted after a system administrator has modified permissions in a way that the system account no longer has adequate permission to access system files and folders.
Resolving the Problem
Before you can fix this problem you need to figure out what caused it. Did you recently install new software or hardware? This will help out a lot when troubleshooting the problem.
- If you recently installed a new driver, uninstall it. If you recently updated a driver, try rolling it back  to a previous working version.
- If you or software you installed recently installed a system service, try disabling  or uninstalling the service.
- A system file mismatch caused by partially restoring the system files from a backup may have caused this problem. Some backup programs do not backup files that are in use. You should always use a backup program that is Vista compatible.