Spybot Search & Destroy. I've used it for years on our family's computers and it is just as much if not more useful than AV.
Remember, when you install S&D to immunize your system. This will intercept bad things as they come through and allow you to stop it, like harmful web pages and programs. Also, I recommend everyone dump IE and use Firefox. IE is a magnet for junk. S&D is less critical if you only use Firefox.