You'll need to call Microsoft, if you have all the details they ask and they check out then they will sort you out... if not you might have to buy a new key.
If you're after the key for the OS that you've already got installed and avtivated, it's saved in the registry. Use this freeware utility to retreive it.
If you haven't installed the OS yet, you'll have to get Windows to issue you a new one.