I have to say Spybot is worthless now. Over 1,100,000 detections and it finds nothing--when I know my computer was badly hit. It's been some time since the core program has been updated.
Mbam is good, but superantispyware did bupkiss for me.
But none of these, including Avast, will get rid of some of the nastier rootkit viruses all by themselves.
I know some other progs I can recommend but I'm at work--I can't remember their names, except one, Combofix. Although you're not supposed to run it without direction from an expert, I did so (I believe its default settings are set to do no harm), and I think it DID remove the main virus that had hit me; that is, even before I got directions from geekstogo.com, the redirect had stopped--just after I ran Combofix (I rebooted, too, so it wasn't a case of the virus being removed only to reinfect on reboot). After that, what I needed was help repairing the registry errors the virus had caused, and Mbam and Avast were then able to get rid of the trojans that the rootkit virus had caused to be downloaded.