David's friend has a computer that had Windows 10 installed, but the activation failed. He tried to activate it again, but it won't work. So he went back to Windows 7 and then had a repair shop install Windows 10 for him. Now it's asking for a serial number.
Leo says the guy at the store probably installed a pirated copy, so the only solution now is to start from scratch. David should format the hard drive and install Windows 7. Insert his serial number and update it. One of the updates should be Windows 10. Or, he can download the Windows 10 ISO from Microsoft and install it. It shouldn't ask for a serial number again.