MS KB article: How to troubleshoot a “STOP 0xC000021A” error.
More information required about what occurred prior to the error appearing :-
- Was any software installed?
- Were any Microsoft updates installed?
- Were any drivers updated?
- Was any hardware changed?
- What Service Pack is installed?
Since the error appeared :-
- Can you boot into Safe Mode (press F8 when the system boots, before XP screen comes up)
- Can you boot into “Last Known Good Config” (although if the system logs on this is useless option as it clears Last Known Good)
- Can you boot to the Dell Diagnostics partition (press F12 while BIOS is loading)
- What happens after the error message (e.g. does windows reboot/shutdown? Keep working?)
As mentioned, a re-install of XP in repair mode MAY be an option, but in my opinion repair is a messy option that has many potential issues, and is not to be used in all circumstances. (e.g. An incompatible third-party program has been installed).
How to perform an in-place upgrade (reinstallation) of Windows XP
Do not use a repair or in-place upgrade to resolve a problem with third-party programs, files or registry entries.
Do not use a repair or in-place upgrade if you suspect disk problems.
Do not use a repair or in-place upgrade if you suspect a problem with a third-party device. Determine whether the latest device drivers are currently installed for the device.