If you don't mind spending the time. You can install Windows 7 on the blank drive. When you get to the point where it asks for the key, just click next (don't enter anything). It will allow you to perform the installation, but it will require that you enter the key and activate with a few days... but this won't work for you as you'll get a message indicating your key is for an "upgrade" only. Now, while you're in Windows, launch the Windows installer again from the DVD. This time, choose an "upgrade" installation. When promted for your key, your upgrade key will now work.
Basically, Windows just looks to see if there is an existing version of an OS installed (activated or not). You're using the first Windows installation as the previous version. Microsoft has recommended this method for similar situations.