Hacked computer networks, or botnets, are becoming increasingly difficult to trace as hackers develop new means to hide them, says security experts. Botnets are used to send spam, propagate viruses and carry out denial of service attacks – something that has again come to light with a high-profile attack on The Million Dollar Home Page, a novel advertising website idea by a British college student. Extortion schemes are frequently backed by the muscle of botnets, and hackers are also renting the use of armadas of computers for illegal purposes through advertisements on the Web, said Kevin Hogan, senior manager for Symantec Security Response.