Here's what comes up from a google search. Windows Marketplace: Help & FAQ
I forgot to print out my product key before I started Windows Vista installation. Now what?
Your confirmation e-mail contains the product key for your upgrade. If you can’t find your confirmation e-mail, check your spam or junk mail folder.
You can also find your product key in your digital locker for three days after completing your purchase.
Your product key is a sequence of 25 letters and numbers, in this format: 00000-00000-00000-00000-00000. Windows Anytime Upgrade Frequently Asked Questions
Can I recover my upgrade license file?
Yes. If you need to restore your computer, a copy of your upgrade license file is available from the digital locker website from Microsoft. For more information about the digital locker, see What is the digital locker website from Microsoft?
You should have proof of purchase (credit card transaction, or similar). If it's not in your Digital locker you can prove you purchased it, and there should be a copy of the license in your digital locker.
Good Luck! If you get Ultimate working, make an image of the drive. This feature is built into Ultimate. Then if you need to reinstall again, you can start right from the point the image was made.