Upgrade Help - Key Required?

In a rush to use my new iPad, I had to upgrade my Mac from 10.4 to 10.6 Snow Leopard. This caused my Parallels Desktop 2.5 to cease working. I can upgrade to Parallels 4, but it is asking for my key to purchase the upgrade. I can't find my key because I can't open Parallels (catch 22!). The email I used when I bought my Parallels was my old work email and is now a defunct company.

Any suggestions for how I can find my key? Can I boot my mac from an old OS Tiger Disk and try to open my parallels to retrieve the key that way? Do I only need a key to buy the upgrade and once I install my upgrade it will be able to use my old key hopefully found in my current 2.5 install somehow?

Any help is appreciated!