Marcus Fenix
Level H
Old keys don't work until they are upgraded as a Windows 10 key in MS servers. For that first you have to do a upgrade and activate Windows 10. After that you can do a clean install any number of times without a key, your hardware id will act as key for Activation.
Regarding the setup asking key, while upgradation from Genuine Windows 8.1, it happened with me as well. I used official MSDN iso file downloaded from MS server using following link and my problem was fixed. If your Windows instalation is English internationl, replace en-us with en-gb in following link
http://www.microsoft.com/en-us/software-download/techbench
Thanks boss.....
Actually I found the issue after much trial and error.
I had downloaded the 64 bit iso in the office using MediaCreator on my office laptop which is Windows 7 Enterprise x64.Then I created a USB drive using Rufus and when I tried running the setup.exe from the bootable pen drive on my Home Desktop running 8.1 64 bit it kept on asking for Product Key.
Then reading the Anandtech article I executed the MediaCreator on my home machine which I was planning to upgrade and then created a bootable pen drive directly from the tool.
After that when I executed the setup.exe on the home machine it worked without asking for any key and the same pen drive also worked on my download rig which runs the same OS i.e. Windows 8.1 x64
So I think that while creating an iso/pen drive from Mediacreator you need to keep in mind that you will be able to use the iso/pen drive only on machines which are running the same OS as on the machine on which the iso/pen drive was created using Medaicreator.(which in my case was Windows 7 Enterprise) and hence it refused to work without a key while trying to execute setup.exe on 8.1 machines.
Now I will be upgrading my Transformer convertible(running 8.1 32 bit) the same way over the weekend.