I recommend Malwarebytes Anti-Malware regularly. I like to call it the “biggest broom” for sweeping out a computer that has been infected.
I want to alert you to a new tool from the same company, Malwarebytes Anti-Rootkit (HT: TechRepublic). It’s still in Beta, but is quite well along in that testing. Be sure to read both of the linked articles and also the readme.rtf file (mentioned in those documents).