If your computer is acting like it may be possessed by some particularly annoying digital demon, it may be infested with spyware. Happily, there are some excellent exorcists that can be downloaded for free. Spyware collects and transmits data about how you use your computer, so marketers can target their advertising pitches. Some spyware applications launch pop-up ads for products that match the sites you visit or the searches you perform. Others can change Web browser preferences, add entries to Internet Explorer's Favorites list, and transmit sensitive information like passwords. The sneaky programs often cause application crashes and slowed system performance