That compromise means they can be controlled remotely by a malicious outsider, often without the PC owner's knowledge. Computers not protected by adequate antivirus and firewall software are at greatest risk of being corralled into a botnet.
Armies ofare used by cybercriminals for sending spam or viruses or committing denial-of-service attacks. Capacity on botnets is rented out to criminal gangs or individuals for as little as $100 for a couple of hours by their creator--often a very commercially motivated virus writer.
A zombie army was used in a high profilethis year, affecting the Web sites of some of its big-name tech clients.
Natasha Lomas reported for Silicon.com in London.