After I upgraded to Win 10, I had to run a utility to find the new key for Win 10. After I completed the clean install, I decided to locate where Windows keeps it. Here is where:
Regedit
Hkey_Local_Machine\Software\Microsoft\WindowsNT\CurrentVersion\SoftwareProtectionPlatform
Click on SoftwareProtectionPlatform and the key appears on the right side next to "Backup Product"
Regedit
Hkey_Local_Machine\Software\Microsoft\WindowsNT\CurrentVersion\SoftwareProtectionPlatform
Click on SoftwareProtectionPlatform and the key appears on the right side next to "Backup Product"