Similar to Neonhomer's suggestion, if you re-install your OS with the factory CD, it may not ask for the cdkey. Like my Dell for example, if I re-install XP Pro using any of the Dell XP Pro CD, it wouldn't ask me for the CDKey.
If you lost your CD key, you have to buy a new one. What you are buying from Microsoft is a key since you can get the media all over the internet, trial, or even thru most of the Microsoft self-study books.