If Windows 8 Developer Preview installation failed during first attempt or if you’re planning to use Reset your PC setting on Windows 8 or reinstall Windows using installation disk, you should know:  you need to enter Windows 8 Developer Preview Product key during installation as the setup asks you. Without entering the product key you can’t proceed forward, Microsoft employee shared the Product key on MSDN forums as answer to this question which you can use.  This key an be used by all users using Windows developer preview.

It’s better to note down this key on win8  DVD using marker pen.  Here is the key.