As per the post above use the Adaware and Spybot software..
They are very good and a free service. Be sure after loading the software to use its update functions.
Beware there are pleanty of scan softwares that offer to fix your system beacause they found hundreds of errors and for a small fee they will clean your system. 95% of these are a waste of money and time.
If you don't have a firewall I would invest in one as this will help you prevent further problems. I suggest Zonealarm it will set you back 20 bucks for a years runtime with updates..
Good luck..