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Q: Knock, knock.

A: Who's there?

Q: Hacker.

A: Hacker who?

Q: [silence]

A: I said, "Hacker who?" Hey ... Where did you go?

It's too late--the hacker's gone. But he's left a little something for you to remember him by. He's messed up your computer, stolen your personal finance records, and sent embarrassing messages to everyone in your e-mail address book.

If you've recently signed up for cable or digital subscriber line Internet access, you're particularly susceptible to hackers. The best way to protect yourself is to install a firewall. Some firewalls are hardware-based; they're actual boxes you purchase and hook up to your PC. Software firewalls are another alternative.

ZoneAlarm, from Zone Labs, is such a creature. It's relatively simple, very effective, and free for personal or nonprofit use. ZoneAlarm effectively bricks up your electronic back door to prevent unauthorized access. It also ensures that you can safely engage in all your normal online activities, including e-mail, chat, software updates, and of course, the all-important automatic downloading of CD track listings for your media player.

Now the catch: ZoneAlarm can be confounding if you're not used to this type of tool. But that's what we're here for. Over the next few pages, you'll see how to obtain your own copy of ZoneAlarm and configure it to keep your PC safe and sound.