Miss it or forget it activation key of Windows you are using? Below we will see how you can recover the product key from an installed Windows 10, Windows 8 or Windows 7 operating system, with one command.
It is the simplest solution to find your product key from an operating system system Windows that is already installed on your computer without using any third-party software. 
You should open a window linecommand prompt and run the following command.
To open a command prompt window search for “cmd” or “cmd.exe”, and open the black icon that will appear first in the results searchs.
Type the following command:
wmic path softwarelicensingservice get OA3xOriginalProductKey
It will show you the product key of your operating system.
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Update:
If the above command does not work on your system, open a notepad and copy and paste the following text:
Store it on the surface work of your computer with the extension .vbs. You can use whatever name you want, for example iguru.vbs.
Below you can download the file. Open .zip and double-click the file iguru.vbs. It will tell you the key of your operating system.
