Right-click on the Start button
Select "...command (admin)"
You should see a DOS (black windows with white text) window pop-up
there is a C:\windows\system32> with a blinking cursor
Just copy this line
wmic path SoftwareLicensingService get OA3xOriginalProductKey
and paste it after the cursor followed by the Enter key
And tada... you will find your windows activation key.
Copy this number on paper.... (you can't copy paste it)
Then go under your windows parameters select Update and security and then select the "activation" option on the left hand menu. Follow the steps.