Is there a "COA" sticker on the case somewhere that includes the OEM key for XP? if so, all you need is an XP OEM (Probably XP Home) installation CD.
If you can't reinstall from scratch, use another machine to download the Kaspersky Rescue ISO and burn it to CD. Boot the affected PC from this CD, do a full scan and fix everything that's found.
The Falcon Four boot CD will allow you to update and run MBAM, SAS and others from it.
Be warned that whatever you use, if operating system files are infected and untreatable, you will need to reinstall anyway.