I did a little digging around and it seems to be coming up with the same for most of the sites I have visited.
This is proberly the most useful post I could find on the subject -
"I'm pretty sure this is a resource limitation, probably not enough RAM. I think it also happens if there is not enough VRAM on the video card.
You can disable the warning forever though by getting Properties on the .exe game file, and set the compatibility mode to disable Aero while it is running. That way you can use your computer normally, when you play it's off, and no more badgering."