My brother's laptop has been offline for a couple months due to a mix up in ordering a new charger. Upon starting it back up after finally getting said charger, Windows is having issues starting up. It's attempting and failing to diagnose the issue using the Windows Startup Repair. (I suspect this might be due to a corrupted registry, but cannot confirm) My brother doesn't care at this point if I delete everything and start anew, however, I am aware that his laptop originally came with Windows 8 OEM and was since upgraded to Windows 10. I am also aware that the product key is embedded in the UEFI/BIOS. My question is, if I do a fresh install of windows will the product key activate an installation of Windows 10 or do I have to go back to Windows 8?