The Windows product key is a 25-character alphanumeric code that’s needed to activate and validate Windows. It ensures that you are using a licensed copy of Windows and not a pirated one. This is important because some of the critical Windows updates are available only for licensed Windows users. If you are already using a licensed copy of Windows, then you need to know where to find it in case you are planning to do a clean installation of Windows on your machine or want to transfer your copy of Windows to a new machine altogether.