Stop:c000021a {fatal system error}

The Internet is full of this error, must be fairly common.
Did you need to supply drivers for your SCSI controller when installing? Raid or SATA configs.

Most fixes say to boot to your XP Cd and run a repair. Also they recommend stopping the virus Scanner when installing patches.
A program called Go Back in roxio can also cause the problem, probably not your issue.

Dell computers were also noted as problematic, probably hardware caused.

Please report back with more specifics. Can you get in safe mode, can you run chkdsk, etc.

unfortunately this BSOD issue is caused by a "security" issue. The only real solution is a reinstall

ref: (NT 4 ref, but still valid)
http://support.microsoft.com/default.aspx?scid=kb;en-us;317189

I have booted a system with the error and removed items from the registry (using ERD Commander) and it's made no difference.

This is a real pain in the arse

This MS article handles the issue a little more specific

http://support.microsoft.com/default.aspx?scid=kb;en-us;156669