According to a recent report from research firm GartnerG2, more than 20 million people have installed adware applications (adware being a type of spyware that reports back on a subject's activities to serve up targeted advertising), and this covers only a portion of the spyware on the loose
PC Magazine has a review of spyware removal tools. From the list of what they reviewed, I have only used Ad Aware and had a good experience with that one.

It looks like Spybot Search & Destroy got the best results in their review.

Here is the full article: Spyware