Windows vista has been a terrible experience so I'm trying to format and reinstall XP. For some reason no matter what I do I can't seem to get the computer to boot from the XP disk. Even with the hard drive disabled it still somehow boots back up to Vista.
Maybe if you share what problem you are having with Vista it may be resolved.
At any rate, the disk can't be disabled otherwise it wouldn't boot. How did you disable it?
How are you trying to format Vista?
To boot the XP disk you need to change the boot order in bios so that the optical drive is the first boot device.
But before you go this route, make sure you have XP drivers for all your hardware, motherboard, video card, and any add ons you may have.
Another problem you may run into if you have a sata hdd, you may need XP drivers on floppy disk or slipstreamed onto the XP disk.