If you’re preparing to reinstall an operating system (for example, doing a clean install of Windows XP) or an application like Microsoft Office, you will need to find your copy of the software’s product key (CD key). Normally this product key is located with the manual or CD that came with your software.
If you’ve lost your product key you can often times find it in the registry but this can be very difficult to impossible to retrieve manually. Luckily, there are many free key finder programs available to help.
Below are the top 3 free key finder programs.
more and source: https://www.lifewire.com/free-product-key-finder-programs-2625119