I'm finally done with my co-worker's computer after six days of on and off work and it's going home.
It runs pretty smoothly now and loads up fast enough, given its age. If I had a copy of the original Windows XP Professional install CD (I KNOW that the guy who installed XP on the machine used a illegitimate copy) and knowledge of what files to back up I might have just wiped the entire hard drive. I didn't have any of those so I went with the slowest option, painstakingly removing every virus, trojan horse, adware and spyware infection that I could find given what tools I had to work with. I removed about 70 virus and trojan horse infected files and nearly 1000 articles of spyware and adware. The computer was so bad at one point that neither the keyboard or mouse would work.
After cleaning the main account in XP, I found that infections were hiding in other accounts, so in the end I deleted those accounts in safe mode. I removed every rouge spyware infested game, toolbar accessory and file that I came across. In the end I set the default internet browser to Firefox 3.0 with add-ons such as adblock plus, no script, KeyScrambler, and Web of Trust. I also found interesting add-ons for IE 7 including an ad blocker, KeyScrambler and Web of Trust. Additionally, I set the the security on the browser to fairly high while disabling pop-ups, just in case they decide to use IE after all.
For the finishing touches I did blow all of the dust out of the computer and wiped off the case and keyboard since they were also dirty. I just hope this computer doesn't run into anymore problems and that they heed my advice on how to avoid really bad spyware and virus infections.
Man, that was a time consuming process, I hope I don't have to do it again any time soon.