Go to your local computer store, tell them that u have a key (you may need to bring the key/computer it's attached to to prove this) and they should burn u a copy for a few $$$. You may also be able to call MS, give them your key, and pay a few $$$ to get a disc, not sure if that will work. Bar that there's always torrents..
Better yet I have a copy of XP Pro I could burn u, if you wanna talk about a few $$ for disc/packaging/postage.