SpyBot-S&D searches your hard drive for so-called spy- or adbots; little modules that are responsible for the ads many programs display. But many of these modules also transmit information about your surfing behaviour and more to the net.
If SpyBot-S&D finds such modules, it can remove them - or replace them with empty dummies in case their host software won't run with its bot removed. In most cases, the host still runs fine after removing the bot.
For a list of 'supported' bots see the feature lists below. The Spybot-S&D interface is so easy, that updates just require replacing a file of about 80k in size. Those updates are distributed by my software mailinglist, at the authors page and are also available from inside the programs update section.
Tools and Utilities
Product page: here