Sure you can. You don't even need to write down the key for W8.1, the system will activate during installation if you have internet connection.
No @NetBandit, it's not legit.
@YodaAteMyCar If you have the restore media/correct media, do a new install as suggested by @JuliaOnly and @Adobe_Flash_Player, backup your files and do the "remove files and clean the drive" option when installing.