XP keys aren't free. And they depend on what XP it is (Home, Pro, Media Center), if it is OEM or retail, and the service pack level.
The best thing you can do is find an unused XP CD with license key on eBay or such a shop. Or you can see if Microsoft still sells them if you find a retail CD with a key that is in use (so can't be activated) and you call their free activation issue phone. But it could well be the don't do it any more, because XP isn't fully supported anymore.
i want XP key plez if any one have then shear it.