In all likelihood, however, you willingly installed much of your spyware yourself when downloading another application. Most file-sharing services—Napster-like tools for trading MP3s and other files across the Internet—are bundled with spyware. That's how file-sharing vendors make money while not charging for their products. In a sense, you are paying, but the coin is privacy, not money.

The Grokster service, for example, includes Gator eWallet. This program seems innocuous. It can learn user names, passwords, credit card numbers, addresses, and so on, to help fill in online forms. But it sends information about you, your computer, and your online behavior to Gator's Web site