Cause
The Stop 0xC000021A message occurs when Windows XP switches into kernel mode and a user-mode subsystem, such as
Winlogon or the Client Server Runtime Subsystem (CSRSS), is compromised and security can no longer be guaranteed. Because
Windows XP cannot run without Winlogon or CSRSS, this is one of the few situations where the failure of a user-mode service
can cause the system to stop responding


Solution

• Perform a Driver Rollback in case the customer has updated the driver for a specific component.
   
• Perform a System Restore from Safe Mode with command prompt
   
• Reinstall the Operating System.