Last weekend I performed an experiment. I tried to re-install Windows 8 and see if it would activate automatically used the OEM key that's embedded in the machine's BIOS. The result was yes and no. Background: Back in 2013, I bought an HP Envy DV7 laptop with Windows 8 Core pre-installed. When Windows 8.1 was released, I upgraded for free using the Windows Store. Later on, I download a Windows 8.1 Core .iso and performed a clean install in order to get rid of the HP bloatware. Windows 8.1 Core was activated automatically without me having to do anything. Using Belarc Advisor, I found that Windows 8.1 was using the same OEM activation key that Windows 8 had used. In July 2015, I upgraded to Windows 10 Home for free using Windows Update. I waited about a week or so, downloaded a Windows 10 Home .iso, and did another clean install. Again, Windows 10 activated automatically, this time via the digital license. Using Belarc Advisor, I found that a different activation key was now being used by Windows 10. I assumed this change was part of the digital licensing process. To the present: After a long series of Google searches, I finally found a download for Windows 8 Core x84_64. (The filename is en_windows_8_x64_dvd_915440.iso). I did not enter an activation key during the installation, since I assumed it would use the OEM key for Windows 8. After installing, I checked and found that Windows 8 was indeed activated. But it didn't last long. After I installed all the updates, an initial batch of 131 followed by a second batch of 23, I checked again to see if Windows 8 was still activated and found that it no longer was. The error message was as follows: Error code 0xC004C008: This product key cannot be used. It is being used on another machine. So what happened? I am puzzled as to why the machine's OEM activation key was initially accepted and then rejected. I am also wondering if MS somehow blacklists your machine's original OEM activation key if you accept to upgrade/install Windows 10 using the digital license. All comments are welcome. PS: I have since re-installed Windows 10 Home using a backup image. It is still activated.