For those of you with ATI cards who cannot change your monitor type from anything other than the default....I may have a solution for you. It's worked for myself and one other person I know had the problem.

Start the hardware wizard.
Choose Uninstall/Unplug a device.
Choose Uninstall a Device.
Click on Show Hidden Devices.

In my case I found four default monitors, the previous monitor I had been using, and a plug and play monitor. I removed all but the Plug and Play monitor entry (painfully, one at a time) and rebooted - I could then change the monitor type in display properties.