Well, that all sounds odd. On Win XP computer, I simply don't have the option to disable the monitor at the Adapter screen. And, that makes sense. Why would Windows allow you to disable a device that is required to use your computer?
Do you have a video card installed or are you using onboard video? If you're using a video card, does your computer have onboard video capability? Do you have any old or extra video card that is not being used? Is your video card AGP or PCI Express?