Windows 8 installation is very easy, the OS will be installed without asking users to enter a product key during installation and in Windows 8 (developer) OS you will not find the product key in a text document in the installation files of the OS.

Update [31/05/2016] : If you are looking to download  Windows 7  / Windows 8 ISO images easily, then use this free & tiny tool called Windows ISO Downloader.

Windows 8 Consumer Preview Product Key : All users must use NF32V-Q9P3W-7DR7Y-JGWRW-JFCK8 product key to activate Windows 8 Consumer Preview.

 

The Problem arises when you need to re-install Windows 8, where users must give a product key to go ahead with the re-installation of OS.To provide a solution to this problem,a Microsoft employee revealed Windows 8 Product key at MSDN forum.

Windows 8 Product Key : 6RH4V-HNTWC-JQKG8-RFR3R-36498

If you’re running a server version of Windows Developer Preview, you can use this product key:
4Y8N3-H7MMW-C76VJ-YD3XV-MBDKV

Note: Above product keys are only for use with the Windows Developer Preview version of Windows 8.

Update 1: Windows 8 Consumer Preview Product Key:DNJXJ-7XBW8-2378T-X22TX-BKG7J
update 2: Windows 8 Product Release preivew product Key: TK8TP-9JN6P-7X7WW-RFFTV-B7QPF