Recently I finished upgrading a PC from Windows 7 to Windows 8 Pro. I had downloaded an upgrade of Windows 8 Pro using Microsoft's Windows 8 upgrade offer that gives current Windows users the ability to purchase an upgrade to Windows 8 Pro for $39.99 until January 31st 2012. I had also bought a new SSD hard drive for the same computer and I thought I could just do a clean install of Windows 8 Pro on to the new SSD using the upgrade disc.
I installed the new SSD drive and put in the Windows 8 Pro Upgrade disc. The upgrade instructions asked for a product key which I entered, after which Windows 8 installed and I was able to boot into Windows 8 Pro. All was well until shortly afterwards when Windows 8 Pro said to enter a product key again in order to be able to personalize Windows. I tried entering the same product key that I had entered when I installed Windows 8 Pro but Windows 8 Pro said the product key was not valid. After several tries making sure their was not any typos I realized something must have gone wrong with the install.
I did some Googling and discovered that to do a clean install of Windows 8 Pro using an upgrade disc you had to start the install procedure in a current Windows operating system. Some people said that you could try to re-install Windows 8 on top of Windows 8, but I decided to just bite the bullet and do a quick re-install of Windows 7 and then install Windows 8 from within Windows 7. Doing this worked out perfectly, without any issues so far. Windows 8 asked for the product key on install but so far has not asked for it again.
Even though you have to start the installation procedure from within another copy of Windows in order for the upgrade version of Windows 8 Pro to work properly, you can still do a completely fresh install. To perform a completely fresh instal just choose to keep none of your existing settings and files when asked by the Windows 8 installer, and then the Windows 8 installer will perform a clean upgrade.