UPDATE: Since I posted this yesterday, I heard from a friend who is a system admin at a large company. She recommends a different set of steps. See below.
Recently (Feb 2010) some acquaintances who are experienced users have gotten a computer virus. It comes through a pop-up window, and it's very hard to remove! I'm not sure EXACTLY how to avoid it, but here's my best advice:
When surfing the internet you may suddenly encounter a pop-up screen that says your computer is infected with a virus. It looks like it is scanning your system. It may also ask whether you want to continue the scan.
* Do NOT click ANYTHING on the pop-up window!
* Do NOT click the X to close the pop-up window.
* Do a control-alt-delete and end the task.
* Download malwarebytes' anti-malware software and follow the instructions.
My friend the sysadmin says she's fixed several computers that were infected with malware.
Here's a New York Times article the addresses the same (or a similar) problem. Let's all be safe out there!