Could you please let us know some more information on your PC;
What is the operating system ?
Is it a laptop or a desktop PC ?
At the time the message appears , do you have any external devices ; like a USB connected to the PC ?
I have a Dell XPS desktop running XP Media Center Edition. The only thing plugged into my USB ports are printer, sensor for media center remote, and sensor for remote keyboard and mouse. These have always been there since new. There is nothing in any of my card slots, DVDs, or floppy drives. Last Thursday I got a pop-up saying there was a remote storage device attached (there wasn't) and did I want to manually scan to make sure it was safe. I did and it was. Since then it does the same thing several times a day without being prompted. At the same time several things went wrong, I had registery problems, lost my google toolbar, and IE slowed to a snails pace. I was able to resolve all those problems along with a clean-up, but the manual virus scan thing just keeps going on and on.