Look for your XP cd. Once you find it, boot up with the XP CD in the drive and let Setup run. Go past the "C" and "R" repair options by pressing Enter just as if you're doing a clean install and you'll eventually get an option to "Repair an existing installation". That's the one you want. If it asks about formatting you've gone wrong, so start over.
You'll get to keep your existing apps & settings, but you need to re-install SP2, Windows Updates & any unsigned drivers afterwards. NOTE: Be aware that you'll lose any protection from Blaster / Sasser type worms. Don't go online without a firewall, whether the built in Windows one or 3rd party.