NEVER agree to buy any software that says they will clean up your PC for a price. They often lie about how many viruses you have, just to frighten you into paying.
You could try Adaware (free) to see if it will find it.
http://www.lavasoftusa.com/products/ad_aware_f ree.php
Also Spybot Search and Destroy has a good reputation (not one I have used though)
http://www.safer-networking.org/en/index.html
Sometimes you need to run 2 or 3 of these programs to find the culprit as not all programs find all threats.