Well, that depends on what they're infected with. For antivirus, you can use NOD32, AVG Free Edition, or any other anti-virus. I would go with AVG(because it's free, and it's a terrific product). Also, for spyware/malware, you should check out some if not all of the following:
Spybot S&D -- Free
SuperAntiSpyWare -- Free
Adaware -- Free
Hijackthis -- Free
95% of these can be downloaded at http://www.download.com
If you run hijack this, then you can start a new thread an paste your logfile and have other people tell you what is good and what is not. Once you get it cleaned out, you should most definately make a backup copy of all your IMPORTANT files, that way, having complete data loss on a HDD isn't AS big of a deal. But I do know what it's like...I got nuked once and the nuke deleted all of my fonts. And guess what a computer can display if it has no fonts...(that was back in the day though). Anyway, let me know if you have any other problems.