yeah, I knew what you meant. I just assumed that had you legitemately purchased the cd and actually called microsoft, their customer support staff would've definitely had you write down the number, (e)mail it to you, or something similar if that was the only time they'd give it to you. otherwise, I don't see why you wouldn't be able to call in and get it again...should the need arise.
a side note, just looked through my registry briefly...now, I'm not saying this IS the code...but it looks a lot like it. take a look and you tell me:
hkey_current_user > software > microsoft > windows > current version > installer > upgrade codes