Experts attribute the global upswing in spam to technological innovations in the way the junk mail is sent. Spam-fighting software is good at rejecting mail from servers that are known to disseminate spam, but spammers are getting better at setting up botnets--networks composed of broadband-connected PCs that, unbeknownst to their owners, are used to send spam. PCs that are directly connected to a cable modem or a DSL modem are particularly at risk of being commandeered.

How do you keep your PC from being shanghaied into a botnet? Make sure you have a good firewall and a secure browser (Internet Explorer users should upgrade to IE 7, which has better defenses, or try an alternative browser like Firefox--see this month's Privacy Watch for more on browser security). If you're on a home network, your router should be protecting you from intruders; check your documentation to confirm that you're taking advantage of the firewall features included with most home routers these days.

Botnets aren't new, but spammers have become more adept at distributing and concealing them. The latest botnet software, which you can run afoul of simply by visiting a rigged Web page or by clicking on a spam message itself, remains inconspicuous by transmitting only a small volume of spam at any one time. Spam-filtering software has trouble distinguishing bot-generated mail from legitimate messages sent by the same computer.