It sounds like something out of a science fiction movie, but programming crooks are gaining control of millions of personal computers and turning them into their own personal PC army, without the owners' knowledge.

WHAT'S A BOTNET?

Here's how it works: a home computer without firewalls and virus protection is attacked by a sophisticated computer virus, known as a "bot." What makes this cyber attack so insidious is the presence of a "bot master" - someone who takes remote control of the computer. Connect all these zombie computers together and you have a powerful network called a "botnet."

The botnets are used to send out spam or to cripple Web sites with bursts of Web traffic. Botnets could also be hired out to anyone who wants to create internet mischief. But what's even more troubling is how easy it is for botmasters to steal more than your computer; they can also steal your identity. Professor Edward Felten of Princeton University said botmasters can intrude into your computer and watch the keys you press -- keys that could unlock the secrets to your social security number, bank account and other personal information.

Experts say there are ways to tell if your computer has been taken hostage.

According to Bill Reitinger from Microsoft, "Look to see if your computer is running more slowly. There could also be hard drive activity when you wouldn't think there would be any."

In addition, computer users may see more pop-up ads.

TIPS

The best way to protect your computer is to stop the infection before it starts. Experts recommend that you install firewalls and antivirus software on all your PCs. In addition, make sure to download the most recent program patches for your software. Hackers often exploit security flaws in software to take over your computer.

If you suspect your computer is a zombie, run an antivirus software. If you're a Windows user, authorize updates on a monthly basis. In addition, Microsoft offers a malicious software removal tool, free of charge. Just go to www.microsoft.com/security. If nothing helps, experts say you may need to wipe the hard drive clean and start from scratch.