http://lifehacker.com/5984278/how-to-do-a-clean-install-of-windows-8-with-an-upgrade-disc
Install Windows 8 as a clean install using the ISO you have. Either try putting the key in during the install, if it works great. If not, don't put it any key during the install process it will install as a trial. Then, type your key in windows to activate. If you get an error, do the registry edit above.
Actually, since you would then need to install ~80+ updates to 8, and then get another huge update (3.2GB), it is better to do what is said here:
http://forums.anandtech.com/showpost.php?p=35622618&postcount=10