The seemingly endless spate of worm infestations over the last year has left something even more troubling in its wake: armies of zombie PCs that can be used to send spam, attack websites and generally wreak havoc over the internet.

Worms such as Sobig, MyDoom and Bagle have been identified as containing malicious code (malware) that allows remote attackers to take over infected machines, while their victims remain blithely oblivious.

UK security firm Sophos estimates that 40 percent of spam is now sent by so-called zombie machines. Sandvine, a network security firm, puts the figure at 80 percent.

Distributed computing company Akamai blames zombie PCs for a DOS (denial of service) attack that briefly blacked out sites such as Google, Microsoft and Yahoo last month. Reuters reports that British teen hackers are hiring out their zombie networks for around $100 an hour.

Besides relaying spam and launching DOS attacks, a zombie machine can be used to send phisher scams, spread viruses, download pornography and steal personal information, says Sophos security consultant Carole Theriault.

"Basically, it is a complete invasion of privacy that can leave you penniless, can have your computer send out all kinds of nasties to other innocent computers and, as part of the collective, contribute to the cyberhavoc going around," Theriault says.

Determining whether your PC is a zombie isn't always easy, says Fred Felman, security software maker ZoneLabs' vice-president of marketing. Symptoms can include a suddenly sluggish broadband connection, excessive hard drive activity, an unresponsive mouse or keyboard, or bounce notifications in your inbox from people you hadn't tried to contact. Yet your PC could show all these symptoms and still not be infected.

You can reduce the risk by installing a personal firewall and antivirus software and keeping your Windows Updates up to date. Yet most home users remain woefully unprotected. A study conducted in May last year by the National Cyber Security Alliance found that two-thirds of home users did not have a properly configured firewall.

Later this summer Microsoft plans to release XP Service Pack 2, which will feature a beefed-up firewall and other security enhancements designed to reduce remote access to PCs. But Steve Gibson, president of Gibson Research Corporation fears widespread adoption of SP2 will cause new problems by creating a single point of attack for malware to defeat.

Even security-savvy users are at risk. Zone Labs' Felman says his own notebook was infected by the Sasser worm while he was attempting to uninstall one firewall and install another. He says users need to take a neighbourhood-watch approach to fighting malware.

"We're all responsible for looking out for weird behaviour in airports and our neighbourhoods; we should also be looking out for weird behaviour on the network," says Gibson, "and we need to start by looking at our own machines."