If you bought a PC with Windows preinstalled, you don't need to enter a product key when you set it up for the first time. The company that built that PC entered the product key as part of the process of preparing the system for delivery to you. Big-name OEMs embed that product key into the BIOS. Smaller system builders enter the product key using deployment tools. In either case, once you start up your brand-new PC, accept the license agreement, and activate your copy, that product key is no longer necessary. You can reinstall that edition of Windows on the same hardware as many times as you want, without having to enter a product key.