If you have lost your Windows 8.1 product key, begin by looking on the DVD packaging, the product order or the confirmation email. If you are still unable to locate the product key, and the computer is currently running an activated copy of Windows 8.1, use third-party software, such as Belarc Advisor to recover the product key. If you are unable to recover the key, contact the computer manufacturer or Microsoft and request a replacement key.
The Windows 8.1 product key is essential to activate the operating system on a computer. To reinstall the operating system, you must enter the 25-digit code. While the computer stores the code in its registry upon activation, it is not in a readable form for the user. The third-party software converts the information back into a readable code.
Some computer manufacturers supply the user with the activation key based on the serial number of the machine. Your computer's manufacturer may allow you to begin a clean install on its website, which automatically adds the activation code, while others require you to call to receive the code. Windows can also provide the key by calling its paid technical support line. Microsoft also sells new activation codes.