SpyBot-S&D searches your hard drive for so-called spy- or adbots; that is, little modules that are responsible for the ads many programs display. Many of these modules also transmit information, including your surfing behavior on the Internet.
If SpyBot-S&D finds such modules, it can remove them. In most cases the host still runs fine after removing the spyware/adware.
For a list of 'supported' targets see the feature lists below. The Spybot-S&D interface is so easy that updates require simply replacing a file of about 80k in size. Those updates are distributed by my software mailing list, and are also available from inside the Programs Update section.
Another feature of Spybot S&D is the removal of usage tracks, which makes it more complicated for unknown spybots to transmit useful data. The list of last visited websites, opened files, started programs, cookies, all that and more can be cleaned. Supported are the three major browsers Internet Explorer, Netscape Communicator and Opera.