Problem with "updating" Windows 7... from Windows 7.

Hello,

I just bought a new MacBook Pro with Mountain Lion and I just installed BootCamp and Windows 7 Professional. The problem I have is that I bought an upgrade version of Windows 7, so I need to install Windows once without entering the product key, then, once Windows is installed, I need to install it again, from Windows, so that Windows thinks it really is an update and lets me enter my key and activate Windows (known as the "double install method"/"clean install from upgrade disc"). Everything worked well until I tried to install Windows for the second time. I got the "0x80070005" error, meaning that the install program couldn't write on the disk. This problem is known for users upgrading from Vista to Windows 7 and Apple has published a fix for this. I tried to install that fix, but since it is made for Windows Vista, it didn't work on Windows 7. So now, I'm stuck not being able to install the "update" of Windows 7.

Does anyone know a workaround for this issue?

Thank you very much!

Etienne