Windows is installed and licensed for you when your PC is built. You do not need to install Windows nor enter your license key during the normal usage of your PC.
If your PC is connected to the Internet, Windows will occasionally connect to Windows licensing servers to ensure that your license is active and registered.
If you reinstall Windows, upon connecting to the Internet, Windows should find that it is activated without your having to enter your Windows license key.
If, however, you're being told to re-enter it by a Windows error message, you can find the license key on a sticker on the back or bottom of your PC.
Please check for this, snap a photo of it, then click the Windows Start button, and type: activation
The start menu will find the Windows Activation tool under Activation Settings.
Then, click Change Product Key, and enter your Windows product key into the resulting popup window. Ensure you're on the Internet when you do, so that Windows may activate itself.
Once this is done, close the activation tool and use your PC normally.
If you have any further issues, please reach out to our support team.