reinstall windows xp, lost validation code

HI guys, I have to reinstall windows XP onto my samsung nc10 laptop because I want to sell it, but alas, I've just realized that the label at the bottom has faded out completely so I don't have a window key anymore. The thing is the lappy, when I bought it years ago, came shipped with an original copy of windows xp (I have windows 7 at the moment). What should I do to reinstall a genuine copy of windows xp?
thanks