Malware has gone from being an annoyance to being a destructive and costly threat to the Internet's viability. "Malware" is the general term (encompassing "spyware" and "adware") for code that lodges on your computer when you download software, visit certain websites, or open infected files attached to spam e-mails. Once in your PC, it sends out more spam, dispatches pop-up advertisements, steals your personal information, or tracks and reports on your Web behavior.

Infection statistics are hard to pin down, because malware purveyors constantly shift strategies to avoid detection, and there is no central regulator or common gatekeeper tracking code across the Internet. But a number of private and consumer groups generated metrics in late 2005.