The Internet is a dangerous place. Spyware, viruses and other malware (rogue software code) can corrupt your files, steal your personal data and even turn your PC into a spam-spewing zombie. Although Microsoft and other software vendors have redoubled their efforts to bolster computer security, it's still up to you to stay safe online.

Whether you're running Windows XP or Vista, three security tools are essential: a firewall, anti-spyware software and anti-virus software. That doesn't mean you have to spend a bundle. In fact, there are free choices for all three lines of defense.

Or you could install a software suite, which is easier to manage than a collection of individual applications. You'll pay to install the most popular suite but — surprise, surprise — there is an excellent, free alternative.

• Firewalls. Both XP and Vista come with a one-way firewall that keeps hackers from invading your PC. But a one-way firewall won't stop malware already on your system from broadcasting to the Net. For that, you'll need to get a two-way firewall, which monitors both inbound and outbound traffic. For home users with one computer, a good choice is ZoneAlarm's free firewall, available at www.zonealarm.com. If you want added security for home networks, ZoneAlarm sells an advanced firewall for $40.
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• Anti-spyware programs. The good news for penny pinchers is that you can download an anti-spyware program and not pay a dime. Microsoft's Windows Defender (www.microsoft.com/downloads) is free for XP users (Vista users already have a copy). Defender is easy to configure, and it runs automatically in the background.

• Anti-virus programs. A proven winner is Symantec's Norton AntiVirus ($40), which blocks both viruses and spyware. Or you could load Norton Internet Security ($70), an all-in-one security suite that includes a two-way firewall. The suite also inspects banking, retail and investing sites to make sure that they are not fakes. Critics have slammed security suites for dragging down PC performance. But Norton Internet Security 2007 is faster than earlier versions.

And, yes, there is a free suite. AOL's Safety and Security Center (daol.aol.com/safetycenter) provides all the protection you need, as long as you sign up for a free AOL account. One caveat: When the security console is open, you'll have to view ads in a bottom window. The ads, however, are easy to ignore.

For casual users who don't regularly frequent the dark side of the Net — which is a nice way of saying gambling, porn and other questionable sites — a suite may be overkill. Says Richard Vamosi, CNET's senior editor and security columnist: "If you don't go to risky sites, you can probably get away with minimum protection."