More details
I'd need to know a bit more about your system. What kind of video card, resolution/refresh rate, and driver version. Its likely that you may need to set your resolution a bit higher, or you may need to reinstall the video drivers. Check the manufactorers website for the most current drivers.
Hope this helps, post back w/ your system specs if that doesn't work.
Hasta,
C0d3Warr10r