After upgrading your Windows 7 or Windows 8 installation to Windows 10, you might be wondering what product key you will use in case you want to do a clean install of Windows 10. If you try using the Windows 7 or 8 key during a fresh install, it will not work. After you upgrade, that key is not upgraded itself so that it will work on Windows 10 clean installs. What happens is that you're secretly assigned a new product key for your new operating system, and you need to find it before you initiate a clean Windows 10 install.
You will need a small utility called ProduKey. It's exceptionally popular and it lets you find the activation key for over 1000 different programs, including a Windows installation. Simply download the utility and run it. Depending on the version of Windows 10 you have installed, you will see the product key listed next to its name when you run it.
Next, create a Windows 10 installation disc or USB stick and run it on your system. When prompted to enter the product key, enter the one you found via the ProduKey utility and it will be authenticated. You can then proceed with installing a fresh copy of Windows 10 on your device.