Brian recommends Malware Bytes and SpyBot Search & Destroy, and a fairly thorough process. First off, disable any virus scanners, and get rid of (or at least turn off) MacAfee, as it's worse than useless. Then:

1.) Download both and open them both up to make sure they're updated.
2.) Unplug your Internet connection.
3.) Run the first one, then reboot machine.
4.) Run the second one, then reboot machine.
5.) Repeat 3 or 4 to make certain that they come up clean. If not, continue repeating until you do.
6.) If something keeps coming up no matter how many times you scan/reboot, comandeer someone else's computer to Google it and research the software that's undoubtedly been written specifically to remove it.

Note that all of this will take many hours, is a pain in the butt, and still might not solve the problem. People used to call J&W with this problem all the time, and when Brian explained to them what malware-cleaning involved (and the costs of his time), they usually just opted to buy new computers. Still, if you've got the time it's worth a shot.

Also, because as a MacBook owner I'm morally obligated to say this:
7.) Next time, buy a Mac. :P