Despite all of this talk of Win32 being "power hungry" and "inefficient", actually Win32 goes out of its way to avoid redrawing (one of the most power-hungry things your computer does) parts of the window that haven't changed.

 

Well, since Windows Vista DWM is optimizing out WM_PAINT messages. If you move your windows around or overlap it with other windows won't throw any WM_PAINT message.