Answer
This question was answered on March 15, 2006.
Much of the information contained herein may have changed since posting.
Install your operating system, windows xp again. Don't do a reinstall, install the operating system as if you are starting over. Go ahead and format the hard drive, that will erase the hard drive. Once you insert the windows xp cd, it will walk you through the process. Just make sure your computer is set to boot from cd. While starting your computer, hit esc or delete or F1 or the other command keys that should take you to you bios. Check under boot order to make sure your cd is listed first.
This should be done yearly anyway.
Then after you have windows xp installed, download microsoft anti spyware from microsoft.com and run it weekly or whenever you need it. It is free.