Spyware fail

Quite often I get calls from frantic family members and friends, reporting that their computer has become infected by viruses and that they've been told to download a program to clean up the problems.
In many, if not most, cases, the message is bogus, delivered by an unscrupulous web site owner who wants you to download a program that, in all likelihood, is going mess up your system.
For example, I stumbled upon a site this morning that showed this screen:

The site I was visiting was momentarily hijacked by a rogue company and presented a simulated virus scan of my system. The gotcha, however, is that I'm not running Windows. (I use Linux.) The whole scan and virus report was made up in the browser. I was repeated asked, via some annoying dialog boxes, to download a program. (The owners quickly got control of their web site. Later visits to the same page didn't bring up this fake scan.)
If you happen to encounter something such as this, you may not be able to stop the download of the program, but you don't have to and shouldn't run it. Close your browser and delete the file that you might have downloaded.
If you did run the program, here are some steps that you can take to remove it: How to remove Personal Antivirus (Removal Guide).
There are a number of very good and safe anti-virus and anti-spyware programs available for Windows computers. If you have them installed, be sure to update the program and problem or virus definition files and then run a full scan as soon as you can. If you don't have an up-to-date set of security programs on your Windows system, I'll be glad to provide some recommendations.

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Update: 9/14/09 - New York Times pwned to serve scareware pop-ups