"As you may be finding out, perhaps half way through reinstalling Windows 8, you must have your product key to continue the installation. Windows 8, as well as most operating systems and other software, require the entry of unique product keys, or sometimes serial numbers, during installation.
Your Windows 8 product key should be with the email confirmation you received after buying Windows 8 for download, or with your packaging if you purchased it in a retail package. If Windows 8 came preinstalled on your computer, your product key should be on a sticker on your computer or with your documentation.
Fortunately, if you can't find documentation of your Windows 8 product key, you may be able to extract it from the Windows Registry using a program called a product key finder.
Follow the steps below to find your Windows 8 product key using one of these tools."
Microsoft has moved to a new activation method in Windows 8 which means you won’t find the typical COA sticker (Certificate of Authenticity) on the back or bottom of your machine. Instead, the Windows license serial is part of the BIOS/UEFI, which Windows will automatically pull from the chip upon the first installation as well as subsequent reinstalls (i.e. when you perform a full Windows 8 recovery). If that, for whatever reason, doesn’t work, you can easily retrieve the product key from Windows 8 using Magical Jellybean’s Keyfinder.