To find the Windows Product Key embedded by the OEM in your BIOS simply open a CMD windows as Administrator and issue the following command from the prompt:
wmic path SoftwareLicensingService get OA3xOriginalProductKey
Run from the C:\Windows\system32 folder.
C:\windows\system32\wmic path SoftwareLicensingService get OA3xOriginalProductKey
In Windows 10 you can cut and paste the command into your command window.
Get into command prompt by searching for it or doing the Windows key and R button and typing cmd
(it should pop up). Then type this command in exactly as is:
wmic path softwarelicensingservice get OA3xOriginalProductKey