If it's caused by adware, you can download free adware removers via the links below. I suggest you install, update, and run them both. Hopefully that will fix it.
"Spybot Search & Destroy" from here:
http://www.safer-networking.org/en/index.html
"Adaware SE Personal Edition" from here:
http://www.lavasoftusa.com/products/...e_personal.php
If that puts things right I recommend you also delete all your System Restore points so as not to inadvertently restore the problem if you need to use System Restore in the next few weeks (same should be done after cleaning up a virus infection). To delete all restore points, Control Panel->System->System Restore. Tick box to "Turn off system restore" and click "apply". All restore points are deleted. Now untick box again and click "OK" to put System Restore back on. It will start afresh with a clean slate.