whoknows

Hi,
i want windows xp for my netbook, i have downloaded and created my own light version but now i need a genuine product key so i can pass all the validation checks etc, does anywere know were i can buy just the key? im guessing it would be cheaper than buying the whole thing.
many thanks.
the netbook currently is on linux, i hate it, cant get things to install, it's very easy to use and fast i must agree with start up taking a few minutes but it just does not seem as stable and i cannot get all the things i had on windows on linux.