well, let me give it a try. Here is my guess. You had Vista on your computer and then you got another drive and installed XP on the next drive. Is that correct?
Or maybe it was this way. You started out with windows XP and then you installed Vista on the second drive.
In either case,if you kept the two systems separated, there should not be a problem.
But problems do happen. When you install Vista, it tries to change the boot loader used by Windows XP so that the boot menu is now controlled by it and not by XP. In that case, it's not a good idea to try to remove the Visa from the system. The XP will have to be reinstalled.
Is that kind of, sort of, maybe what you are thinking?