MS Visual Studio Express Product Key location?

I have a machine with MSVS Express 2008 (the free version) installed. I purchased a license for MS VS Pro 2008 but when I try to install it, the Express product key is in the installer and it is grayed out, not letting me change it.

Can someone please tell me where this product key is stored in the registry so I can snip it out and move on with Pro?