Error C000021A means that the Windows startup process terminated.
If the full error message matches this one:
The verification of a known DLL failed. System process terminated unexpectedly with a status of 0xC0000000 (0xC0000034 0x0010037C). The system has been shut down.
Then Windows is refusing to start because one of the core system libaries (a DLL file) has become corrupted (by a disk error) or altered (by a virus or other malware).
Try recovering system files from a recover partition or using Windows repair mode from the installation disks.
Remeber to back up your files before recovery as some built-in recovery systems restore the PC to the factory settings and delete all user data.