Applications :: Safari Popup - Windows Virus Warning

I know Macs don't get viruses. Here's my issue. I'm running OSX 10.6.3 with Safari 4.0.5. About 5-6 times a day, running Safari, I get the message I'm attaching to this post. It's a fake popup -- obviously fake, because it tells me it's "Windows Internet Security," and gives me a virus warning. The Safari screen behind the image is grayed out, so the message isn't a real popup -- it's part of the screen.

Note:

- This occurs on mainstream webpages, including google search results pages, Yahoo, and the Wall Street Journal.

- The only MS products on my computer are Office for Mac apps, which I've had for years w/o problem. They are not running when the problem occurs.

- I can click close on the Safari window (not the red button on the warning) or the back button, and the msg goes away.

- I cannot click on other items on the page when the message is there.

- [edit] Also, I've reset Safari, clearing the cache and the cookies. It's only been happening for about a week, but it happens very frequently now.

Again, I know macs don't get viruses, but does anyone have any helpful suggestions for getting rid of this?