Well, your best bet really is to uninstall vista then partition your drive, so you have two partitions. install xp on one and completely finish installing xp with all the latest drivers and updates, then with the other partition you can install vista and do the same with updates and drivers.
Its best to do it this way as vista will take the boot manager, just remeber to wait for the boot loader screen to pop up when booting up, so you can chosses what installation you wish to use.
The main reason for this is because xp is older, its alway best practice, with dual booting, to put the latest os on top of the old one, you can do this vise-versa, but it may bugger up your installation's.
Once you have installed all, google BCDedit, download that and install, as this will tell the computer you two os's and then you can choose between them, with out it, the pc will just boot into vista all the time and uignore xp.
Hope this helps
Edit: install BCDedit under vista, little tip aswell, when you install bcdedit under vista, download the tweaker and install that, that way, if you ever forget to choose xp and it boots into vista, you can then choose xp from the menu that will be installed next to your clock, this will allow you to choose xp and it the pc will restart into xp and you wont have to wait for the bootloader screen as it will automaticallly choose xp as it boots. I do believe doing all what i said under xp (install bcdedit under xp and install the tweaker again, this way you can chop and change between with ease.
If this helps, please tip those little ol' scales.
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