Copyright 2007 HowStuffWorksSee more computer images.

Imagine that the Internet is a city. It would undoubtedly be the most remarkable and diverse city on the planet, but it would also be incredibly seedy and dangerous. You could find the world's most comprehensive libraries there alongside X-rated theaters.

Inside this city, you would also discover that not everyone is who they seem to be -- even yourself. You might find out that you've been misbehaving, although you don't remember it. Like the unwitting agent in "The Manchurian Candidate," you discover you've been doing someone else's bidding, and you have no idea how to stop it.

A zombie computer is very much like the agent in "The Manchurian Candidate." A hacker secretly infiltrates an unsuspecting victim's computer and uses it to conduct illegal activities. The user generally remains unaware that his computer has been taken over -- he can still use it, though it might slow down considerably. As his computer begins to either send out massive amounts of spam or attack Web pages, he becomes the focal point for any investigations involving his computer's suspicious activities.

Video Gallery: Spyware and Viruses Computer viruses range from pesky to outright dangerous. Learn how clicking on what looks like a harmless e-mail message can lead to hours of recovery efforts, if not irreparable damage, in this HowStuffWorks video. Learn how to protect yourself against spyware with Spybot -- Search and Destroy from Harvard University’s Extension School.

The user might find that his Internet Service Provider (ISP) has cancelled his service, or even that he's under investigation for criminal activity. Meanwhile, the hacker shrugs off the loss of one of his zombies because he has more. Sometimes, he has a lot more -- one investigation allegedly discovered that a hacker's single computer controlled a network of more than 1.5 million computers [source: TechWeb].

In this article we'll look at how hackers can commandeer your computer, why they do it and the best way to protect yourself from malicious attacks.

A Zombie by Any Other Name Some people think the term "zombie computer" is misleading. A zombie, after all, seems to have no consciousness and pursues victims on instinct alone. A zombie computer can still behave normally, and every action it takes is a result of a hacker's instructions (though these instructions might be automated). For this reason, these people prefer the term "bot." Bot comes from the word "robot," which in this sense is a device that carries out specific instructions. A collection of networked bots is called a "botnet," and a group of zombie computers is called an "army."