Zombie PCs, unlike their human-based counterparts, don't drool blood, moan or feast on human flesh. The digital undead are far less obtrusive: If your PC has been hijacked with malware and becomes a member of a "botnet," it will silently send spam or join in distributed denial of service attacks that disable target sites with a flood of traffic. But here's the bad news: This kind of misbehavior is so well hidden that it's often impossible to detect, even while staring the culprit in the face--or using it to read an article on Forbes.com. A few clues, however, may help root out the zombie on your desktop.
Your computer seems preoccupied.
If your computer seems to be busy even when you're not, it may be living a secret life of crime. Slow performance, or blinking lights on your modem and a whirring hard drive when no visible programs are running, can mean that malware on your computer is sending out spam or worse, participating in a distributed denial of service attack that floods a site with information requests in order to disable it.
You've visited shady sites.
Today, just visiting a Web site can secretly hijack a computer with malware. Just before the Superbowl in February of this year, for instance, Dolphin Stadium's official Web site was hacked to infect thousands of visitors with a Trojan horse that allowed malware to be downloaded to the machines. But while no site is completely safe, the Internet's "bad neighborhoods"--like pornography and gambling sites--are far more likely to infect your computer with zombifying software.
You don't frequently scan.
Anti-virus scanning programs aren't guaranteed to catch every bug that could turn your computer into a spam-spewing zombie--today's most advanced malware mutates into a different form as often as every 30 minutes to avoid detection, according to Internet Security Systems' Josh Corman. But if you skip anti-virus scans or use an out-of-date scanner, you leave your computer vulnerable to a much wider variety of exploitation, and make it even more difficult for your scanner to catch quickly shifting threats.
You skip software updates.
Anti-virus software isn't the only program that has to be vigilantly maintained to keep your computer clean. Software like operating systems and Web browsers can also have vulnerabilities that are intermittently discovered and patched by software vendors. Skipping those updates leaves your computer with the flawed older version of the programs, and sometimes a wide open back door for malware infection.
You use common software.
Today's cybercriminals methodically seek to exploit the most computers in a given amount of time. That means they're more likely to target common programs and platforms. Since PCs have a far greater market share than their Apple counterparts, Apple machines' vulnerabilities are less targeted than those of PCs. Similarly, a commonly used Web browser like Internet Explorer is more likely to attract malware than a less common one like Firefox or Safari.
You have insecure e-mail practices.
Most e-mail users know better than to open attachments on e-mails from strangers, even ones that promise naked pictures of Britney Spears. But just clicking on a link embedded in a spam e-mail can lead your computer to a Web site that allows it to be infected. E-mail programs that automatically download images within an e-mail's body can also leave your computer vulnerable to exploitation whenever you open a message.
You've received a warning from your ISP.
Even if you can't tell that your computer has been zombified, the company that controls your access to the Internet often can. If your Internet service provider detects an inordinate amount of bulk e-mail coming from your account, it will sometimes send you a message warning that your computer may be sending spam. That could mean your teenage son has started an e-mail marketing company for hawking counterfeit Viagra. But more likely, your computer is a hardworking member of a lucrative spam botnet.
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