On the STOP C000021a error: It may be that 1 of your boot files has been
corrupted, or perhaps, worse, your system registry.
Set your pc BIOS boot sequence to boot from CDROM , reboot from XP CD, login as
Administrator or with admin-rights into the Recovery Console.
See this Microsoft article, and scan down to "how to use Recovery Console".
http://support.microsoft.com/?kbid=307654
While in there, run "CHKDSK C: /P" on your system, then repeat chkdsk for
each of your drives.
If you ARE able to login to Recovery Console, that's an excellent sign; and then running CHKDSK (as above) should be enough. Next, exit out of Recovery Console; remove CD. Reboot pc & login to XP.
If no success, repeat the Recovery Console & look for some files.
I'm presuming here, that you know a little bit on how to use a "Command prompt" session (somewhat similar to DOS).
Do a DIR to look for the next 3 files. Then do a "TYPE BOOT.INI" to list its contents onscreen.
Take a look at your XP system drive ---the root folder (usually C:\ )---take a
look at presence & contents of Boot.INI file, and the presence of a couple
of others, NTLDR , NTDETECT.COM.
Take a good look at BOOT.INI file.
The last consideration would be an "install-repair" ---if none of the above
worked out.
See these resource/references: Aumha.org's knowledge base on stop errors
http://aumha.org/win5/kbestop.php
and
http://aumha.org/win5/kbestop.php#0x21a
Try other suggestions first; save this for very last !
Here is an excellent page on how to do a XP "repair-install". But remember you've got to have a full XP CD----not a simple "recovery CD" from a pc manufacturer (other than Dell {who does give you an Operating System CD}).
See Harry Ohrn's page on "How to Repair Windows XP by Reinstalling".
http://www.webtree.ca/windowsxp/repair_xp.htm