This can maybe help you:
After starting a new game try to press F5 to go to fullscreen mode and then F5 again to go back to windowed mode.
I also suggest you try to update your video card drivers by downloading the latest drivers from your vendor's website. If you get a message saying you already have the latest drivers, then you may need to use for example
http://www.guru3d.com/category/driversweeper/ to remove your current drivers.