Terry has a MacBook Air, running Parallels so he can dual boot into Windows. After he upgraded to Windows 10, however, he had to upgrade Parallels and it trashed the drive. So he rebooted and reinstalled everything, and now Parallels wants him to pay for it again. Leo says that somewhere on the drive was a hidden file, perhaps in the application support folder, that has his registration data. So if he formatted the hard drive, Terry lost that data. Leo also says he'll have to reinstall Windows 7 again after installing Parallels. Then once that is handled, he can run the upgrade to Windows 10 again. But he should backup his data before he does that. He should look for the virtual machine folder and copy it.
Another idea is to run BootCamp. Parallels will recognize Windows in BootCamp and run that. Then it will be permanent and he won't have to install Windows 7 first.