LC21, just use DBAN. It's very useful and recommended when dealing with a virus or worm. Then when re-installing Windows just format the drive regularly.
http://en.wikipedia.org/wiki/DBAN
fa48, I don't quite understand what you mean by "previous OS" on an XP-loaded box. I suggest you just move up to Windows 7 by using the ACM deal listed in this section of the forum, Win 7 is a great OS.