You probably have a worm.

Try the easy fix mentioned at the end of point 2 here, it's worked for a lot of people.
WARNING: Viruses on the network you should be aware of

If that doesn't help, there are links to help sites below
http://www.spywarewarrior.com/index.php
http://www.castlecops.com/forums.html
http://forums.spywareinfo.com/index.php?showtopic=79038


Always make sure that you're using a good AV program. Keep it updated & use it to scan files that you've downloaded before you open them.