You should first install the older OS (in this case being XP) then install the newer OS (Vista) when booting it will display a screen asking you which one you want to boot to. I think it lasts 30 seconds and then goes to the new one if you don't select one.
Vista and XP will be able to see each other, but because of a security feature in Vista XP will not be able to see all files and folders in Vista or copy everything, and Vista will be able to see all passworded account files in XP, though you MIGHT have to be a admin in Vista to see passworded account files in XP