What you do--or don't do--to secure your computer at home affects the Internet as a whole. Some people conscript vulnerable PCs as agents for even bigger crimes. Armed with PCs themselves, these users can draw on the resources of hundreds or even thousands of home PCs to attack a commercial or government Web site. In February 2000, malicious users took down eight major Web sites, including ZDNet, which is owned by CNET Networks. This amazing feat was accomplished by what's called a denial-of-service attack , or DoS, which sends thousands of bogus requests to view a Web site's pages. In a distributed denial-of-service attack , or DDoS, such as the one in February 2000, hundreds or thousands of remote or zombie PCs all bombard one Web site, forcing it to become slower or to shut down from the overload. That was then. Now, thousands more home PCs are equipped with always-on cable and DSL, making the potential for a more massive DDoS even greater.

When viruses and DDoS merge
Last year's Nimda worm is a working example of what may come. While it spread, Nimda actually slowed the Internet itself; it was the first such Internet worm to do so in a measurable way. However, Nimda was also designed to take down the White House Web site. That didn't happen, in part because the White House moved its Web address, short-circuiting the worm. Next time, the target might not be so lucky.

In addition to viruses and worms, malicious users sometimes deploy Trojan horses , bits of code that open unused ports on your computer in order to maintain remote communication with someone, often without your knowledge. Trojan horses allow an attacker to take control of your PC, record your keystrokes, find files with your credit card or banking information, and send that information out to others. They also contain code for a future DDoS attack, remaining quiet on your PC until the moment that someone calls the code into action.

No one knows how many DDoS Trojan horses currently live on home computers, turning them into zombie machines, but estimates range from the thousands to the hundreds of thousands. Malicious users are known to sell or trade lists of "owned" computers. One of them could be yours.

Firewalls help
That's why you need a firewall , software that restricts the information that gets in or out of your computer. Whether you use a dial-up modem or have a dedicated DSL or cable modem, it is important to make sure that no information gets into or off of your computer without your permission. Our Editors' Choice is Zone Alarm. Other good choices are Norton Internet Security, which includes Norton AntiVirus and Norton's own firewall.