a lot of it depends on what the laptop is and if there are drivers for xp. if it's all newer components I would say most likely you're going to have something (or somethings) that won't work with xp, and you're just going to have a headache on your hands.
so the question(s) are, why do you want to downgrade (is it just personal preference, or for business) and 2) what is the laptop model.