All I can recommend at this point is to go to another computer and generate a Preinstalled Environment disc. Preparing a PE disc is tedious, but worth every second of your time.
BartPE is a good tool for this. You'll need your original (legit, non-upgrade) Windows XP install CD. Load up anti-virus and anti-spyware plugins and you'll be able to boot from this CD and perform repair and administrative operations from a portable, minimalistic Windows environment.