To find out what is causing the problem get a copy of hijack this (free) from:
http://www.download.com/HijackThis/3...-10227353.html
Then go to the spyware info forums and post the log file that hijack this produces.
http://forums.spywareinfo.com/index.php?showforum=18
Make sure you read the sticky files at the top first. Most of the stuff you need to know is in them.
Someone will usually be able to tell you exactly what the problem is and where to get the fixes.
When you have the computer clean again, there are a few things I would recommend that you install. Do you have broadband?