Ghacks posted a blog entry that Windows 8 asks after a reinstall sometimes for a product key

You find the answer in a forum thread (!) over at the Microsoft Developer Network forum. A user asked the following question on the forum:

I’m reinstalling Windows Developer Preview and need to enter a product key. What key do I use?

A Microsoft employee replied with this answer:

If you need to reinstall Windows Developer Preview or use the Reset functionality, you might be asked to enter this 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: These product keys are only for use with the Windows Developer Preview version of Windows 8.

  Windows 8 Product Key after reinstalling