gah! System Defender

Ok, usually McAfee and Spybot have had no trouble getting rid of bad bugs, however, system defender has them and me stumped. PLEASE help! The bug isn't installed yet, but I believe it has attached itself to several files (which I do not know how to detect) and no matter how many times I run McAfee. System Mechanic, and Spybot. It keeps coming back with the "threat" message and warning me I must update... blah blah blah, which basically means they're trying to get me to actually download it.