Update: You can get the Windows 8 Release Preview product key from this link.
When the Windows 8 Developer’s Preview was launched, there was no product key required during the installation of the operating system. However, things soon seemed confusing when people tried the Refresh/Reset feature of Windows 8, as a Product Key prompt started showing up. In this case Microsoft provided a key to bypass this apparent glitch. Unlike the Pre-Beta version, Windows 8 Consumer Preview does not instal directly without the a product key. You can get the Windows 8 Product Key for the beta version from the end of this post. This Microsoft authorized key will work as long as the next version of Windows 8 (e.g. RTM) is not released.
Before you grab the Windows 8 Beta Product Key, check out the installation instructions. According to Microsoft, the easiest way to convert an ISO file to a DVD is to use Windows 7’s native Windows Disc Image Burner. Windows XP or Windows Vista users can make use of any third-party program which may be required to convert an ISO file into an installable media. on a CD/DVD or USB. We would personally recommend the use of the Windows 7 USB DVD Tool. For instructions regarding Windows 8 installation, check out these posts:
CD/DVD and USB Installation
Windows 8 Consumer Preview Setup
You can also download the Windows 8 Consumer Preview Setup, which includes tools that enable users to create a DVD or USB flash drive from an ISO file in Windows Vista or Windows 7.
The Windows 8 Product key is given below. The same product key is applicable for both Windows 8 32-bit and Windows 64-bit versions. It can be used for the English, French, Chinese, Japanese and all other versions of Windows 8 Consumer Preview.
Windows 8 Beta Product Key: DNJXJ-7XBW8-2378T-X22TX-BKG7J