1) Drop the drive into a different computer as a secondary drive and scan it with a couple of current-generation antivirus products.
free online antivirus scanners Note the precise names of the viruses you find, and post them so we can look and see what types of user behavior might need modifying.
2) copy out any files they want to rescue, and save them on the healthy computer.
3) confirm that you have a full-on, genuine Windows CD to reinstall from, then delete all the partitions on the infected drive.
4) put the blanked drive back in the other computer and reinstall Windows, following
some security measures to ensure it doesn't come down with worms in the process.
5) get the system set up as securely as practical and educate the users about where they went wrong last time. Maybe it was P2P, maybe it was clicking on enticing links in an IM program, maybe it was lack of a firewall/router plus expired antivirus software. Finding out what it came down with will help prevent it from happening again.
If you are dealing with an off-the-shelf computer that needs a recovery partition on the hard drive in order to reinstall Windows, then your best bet might be to try the manufacturer's recovery procedure first. If you have a full-version unadulterated WinXP CD, however, then Drop The Bomb On It :evil: