I have a dell vostro 1520 laptop purchased from USA. with it came a vista cd and xp cd,sp3 both for reinstallation. currently i have xp installed on it and it did not ask for the product key. . I plan to sell the laptop with vista installed on it and want to retain the xp cd so that i can install it on my assembled desktop pc.the bottom of the machine has the product key sticker mentioning vista but not xp. is it possible to use the xp cd on my desktop and use the key on the bottom of my laptop to authenticate it? will it be a legal version then? when i install vista on the laptop, do i have to use the same product key? Will it clash and will both be legal since i will be running two machines, one with xp and the other with vista, but with the same product key.