Virus - System Doctor

Using Safari, I am getting a virus that inserts a tile ad on web pages that asks me to click for info on "System Doctor." I never click on it. After a few moments, however, an Apple window pops up asking if I'd like to download System Doctor. Since cancel is an option on the window, I click it, and the window is replaced with another window that asks if I'd like to download the software. Only this window does not allow you to cancel and you cannot change to any other windows without clicking OK. When you click OK, the Apple download window (i.e. the one with the bar graph that pops up when you are downloading widets, etc), comes on and I click "no" when it asks if you want to download.

Then the System Doctor stuff goes away, temporarlily. It usually comes back a couple of minutes later and I have to go through the whole thing again. Frustrating!

On the Sophos web site it lists System Doctor as a Windows problem. I've got an iMac with the Motorola chip. Any ideas on how to get rid of this?