I can only tell you how I approach these things, you decide if any approach suggested by me or anyone else...makes sense to you and your situation.
The option I would employ...if I chose not to simply delete the install and do a clean install...BleepingComputer.com - Am I infected What do I do - http://www.bleepingcomputer.com/forums/f/103/am-i-infected-what-do-i-do/
Mind you, I am not saying that this particular STOP error signals an infection...but I tend to believe that such is possible and must be disproven...before I extend any time and energy trying to "repair" a situation that may lie beyond repair. The possibility of malware is something I like to eliminate from consideration as quickly as possible.
Reinstalling XP is no inconvenience for me, I have backups, install CDs, and valid licenses...and plenty of time to do such if the need arise.
Others here may offer more palatable options/suggestions to you.