I hope you have not just paid the full price?
Although its an upgrade version, you dont need an os installed. I had win 7 beta, but were able to do a full format as a fresh install.
You buy the key and get this by email, which includes the download link. With this you download all the files and can upgrade or use custom install to do a full and clean install. If you have any problems and would like the iso cd image, contact them and you will get a new link for this image.
Download, burn and now boot from cd, again use custom install to give options like partition and format.
Trust me, i have it and have installed it this way!
So I can buy an upgrade CD, and install it on my empty hard drive? I'm using it for the computer I just built.