Spybot Search & Destroy is and does exactly what its name claims. It hunts down and kills all of that damned adware, spyware and other related malware that finds its way onto Windows desktops.
And once you’re done being appalled at how much crap has taken over your computer, you might want to go find an alternative web browser since Internet Explorer seems to be a nearly-open conduit for intrusive and annoying garbage. (Yes, that can be prevented. We both know that in most cases the end-user is uninterested in taking the appropriate steps. We’re talking about the real world, here.)
I used to rely on Ad-Aware, but when it’s almost half a year between reference file updates it’s hard to justify the bother of running it. (Yes, I know they just came out with a new version. Sorry guys, too late. And Spybot doesn’t want my money for a full-featured version.)
And if you’re not sure what all of the fuss is about, I recommend a little light reading.