Microsoft's released a new version of their Malicious Software Removal Tool. Give your PC the ol' one-two with it to make sure only the nice programs stay.
Note: an update to this program comes out once a month. Windows XP users, save yourself time and get the latest version automatically through Windows Update. In Control Panel, select "Automatic Updates" and set it to, uh, "Automatic."
Malicious Software Removal Tool [Microsoft]