One evening late in 2001, Julian Green's 7-year-old daughter came upstairs from the computer room of their home in the resort town of Torquay, in western England, and said, "The home page has changed, and it's something not very nice." When Mr. Green checked the machine, he found that the family PC seemed almost possessed. The Internet home page had somehow been switched so that the computer displayed a child pornography site when the browser software started up. Even if he turned the machine off, it would turn itself back on and dial the Internet on its own. Mr. Green...