Can you provide machine specifications, including video card and monitor type and native resolution? What drivers are you using? This will help people help you diagnose your problem.
I have a Dell Inspiron laptop with an NVidia GeForce2 Go card, and my native resolution is 1600x1200. Using the native Linux video driver and setting the resolution lower caused the display to have a bunch of black space around the desktop. I had to install the OEM NVidia driver to get it to work properly.