Installed Vista without entering Prod. Key

I have a Dell XPS400 that came with XP PRO Sp2 installed. Recently I purchased the Vista Utimate retail upgrade. I installed a second hard drive and installed Vista on it using the clean boot method of installing it twice, once over itself. I then set out to install programs and numerous drivers and utilities etc. You have all been there so you know the drill. At some point I realized that although I had a dual boot, I had no idea how to remove XP when I was satisfied with Vista. My first post on SysChat was a reply to a very excellent tutorial by William Wilson on removing XP from a dual boot. Not having much experience posting on forums, I did not realize that I should have posted over on this thread.

Today I remembered that although I installed Vista twice I had not entered the 25 character product key either time! Soon I will have to activate it and I don't know any way to enter the key except to reinstall again and lose everything that I have done up to now. I have Acronis True Image 10 installed but I don't see how that would help in this case.

Does anyone have any ideas or suggestions that I could try? My only excuse for getting myself in this mess, is that I was not too smart to start with and I am older than I realized.