Try something like Malware Bytes Anti Malware as a quick and easy fix to remove most nasty stuff. (www.malwarebytes.org)
After that I would recommend AVG Free (free.grisoft.com) or Avast Home (www.avast.com) both are free antivirus products and cant complain about them.
Personally I have found the Avast one to be better recently.
Phil Roberts, July 2009