First, is your old copy of XP a full retail copy or an OEM? If the latter, you won't be able to install it on your new machine - the licence for it dies with the computer, you can't transfer it.
What don't you like about Vista? It's a lovely OS, far more stable and crucially it's more secure.
If you're hellbent on being a Luddite, you'll have to format your hdd before you can install XP.