You're a responsible citizen, right? You wouldn't offer a lift to a fleeing bank robber. You wouldn't share inside information about your company with a stock-speculating domestic goddess. You probably wouldn't even help a slumping big leaguer cork his bat.

But you may be unwittingly helping spammers fling their annoying and often obscene come-ons all around the Internet. And the danger isn't just that you're helping to perpetuate some get-rich-quick scams--you may also get saddled with loads of problems if a junk mailer successfully sets up shop inside your PC.

According to many experts, it's fast becoming standard operating procedure for spammers to "hijack" unsuspecting users' PCs and steal their hardware and network resources to send unsolicited e-mail. The practice is "absolutely epidemic," says Robert Arnold, an abuse investigator at EarthLink. "We block thousands of [hijacked PCs] that are generating spam every day. And when it does happen, most [victims] are completely unaware that anything's wrong."

Spammers aren't the only culprits, either: Researchers recently said that hackers were using similar hijacking techniques to use innocent people's PCs for hosting pornographic Web sites.

Spammers get into people's systems by exploiting holes in the intricacies of network settings that few users worry about. The wrongdoers may find these vulnerabilities already in existence on your system, or they may get you to download a virus that creates a security breach in your PC. Antivirus researchers, for instance, say this summer's Sobig virus was likely created by spammers as a way to gain access to people's machines. Once spammers locate or create a hole, they're in business. And while your machine is sending out thousands of bogus stock tips or invitations to view bizarre versions of animal husbandry, you may notice nothing more than a slight slowdown in your Internet connection.

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