Hi,
defrag will not get rid of any threats. It will only make some space if you have a small hard drive and windows has not been drfragged in a long time. You can get to defrag 2 ways, the easiest is to go to into My computer and then RIGHT click the drive C:\ Now click on Properties and the Tools. Defrag is over rated for use in XP and solves very little, being aleft over from Win98. XP is more efficient at hard disk management.
Sounds though like your problem is spy ware. use a free tool called spy ware terminator available on the web. This will kill pretty much all such infections. make sure you have a good Anti Virus as well. A good free one is the AVG free edition.
-Dave
www.daveshelpline.com