1) Change you AV(anti-virus) to Avast, Avira or some paid variant (Kapersky).
Download you AV of choice before hand and put it on some media if possible.
2) Before formatting C:\ or whatever drive you XP resides on , you might want to consider the following: do you have any wedding pictures or music files that are important and need to be backed up to some external HD?
4) If so, back them up first, and then scan the external HD to make sure there is nothing on it, after backing up all the important stuff.
5) Make sure you have access to all the drivers(sound, video, etc) via online or some CD on hand, cause you will need to re-install a majority of them if XP does not pick some of them up.
6) Backup this file: wpa.dbl found in C:\windows\system32 .
Wpa.dbl allows you to not have to activate your XP again or have issues
activating XP, since you already did it, you just copy this file back into the same folder after re-install.
Method: B (lazyman/I don't want to formact C:\ but have some time on my hands):
1) Physically remove the HD from the infected computer, and change the jumper settings to slave.
2) Install in a working PC, preferably one that is not infected with viruses or a Linux or Unix box and mount infected drive.
3) Use a ton of standalone scanning tools for both malware and viruses to scan the infected HD.
4) When this is done, switch over to some online tools and do the same thing, scan for malware and viruses.
[Disclaimer: complete list of some tools I use for this are furnished upon request.]