On my system, this is a problem if you have a 3D application with running, which takes some time to redraw, and drag any window over it. It causes a very jerky movement. This also happens if you drag a dialog from the 3D app over its 3D window. The application actually gets a redraw message (WM_PAINT?) which causes it to do a full redraw. Shouldn't the background window be cached by Windows as a bitmap or something?
I've pasted the NVIDIA system information dump below, note that I have 2 GPUs. Don't know if that's significant, but we're seeing this problem on another machine in the office, which also has 2 GPUs and Windows 7. Other machines which have 1 GPU don't have this problem.