Can you go in through Control Panel and into Appearance and Personalization and change it that way?
Also check the Hardware and Sound category. View devices and printers.
What is the device name of your monitor. What properties are you seeing for your monitor there?
Check the driver version of the monitor.
If you have ATI Hydravision, it might be screwing up even if it's not enabled. I had it stick my resolution to 600x400, which sucked a huge amount because even basic menus didn't fit and I would be unable to select the OK button. A little more info on your PC would help... but if you have ATI Catalyst Control Center, you should try removing Hydravision.