To reinstall Windows on the same Computer or move it from the existing one to a new computer, it is necessary to know which version of Windows is installed on the computer. Depending on the version one may or may not need the product key.
If Windows 7 is used earlier in the computer, it can be activated by a single product key that is stored in the Windows registry. Using a simple script or third-party software the key can be extracted easily. This key can be used to install Windows again in the same computer or in another machine if the product key is deactivated from the computer where this product key is initially used.
Product Key for Windows 8.1 version it will not be available unless a retailed version of the operating system is purchased, it is stored in an encrypted format in the BIOS or UEFI firmware. The advantage of encrypted format for reinstalling Windows 8.1 Version will be useful even if there is no product key and the disadvantage is it can be used only on the computer which has this encrypted format and it cannot be used in any other computer.
With the latest version Windows 10, there is something called digital entitlement. This is based on the computer’s hardware and is stored with Microsoft. This is true for any OEM Machines that upgraded to Windows 10 version from Windows 7 or 8.
How to store the product key in case there is a necessity to use it again in another computer:
By using third-party program from Nirsoft called ProduKey
This program instantly finds all the Microsoft application installed on the system and gives the Product ID and Product KEY.