Hi
I hate to sound negative but honestly, (depending on the laptops model) it can be a PITA setting up and then installing XP / Vista on a dual boot.

Not that technically it's difficult just that if you don't have all the orignal driver disks for both vista & XP you may get problems. Yes you CAN downoad them, IF you can find them and IF they work and IF all components come with XP & vista drivers.

If you don't have a specific NEED to run XP and you are setup running Vista I would strongly suggest leaving it that way.
If you do need to run XP , then go for it