How do I find out which product key was used to activate Windows?

For Windows 7, 8, 10 you can use the slmgr command line program to reveal a portion of the key to help narrow it down.

1. Open Start Menu, then type:

   cmd

2. Right click CMD application in the search listing

3. Then click "Run as administrator"

4. At the command prompt, type the following and press enter:

   slmgr -dli

5. Wait for a little window to appear and then look at the value to the right of "Partial Product Key:"

   

6. Compare with and match to your windows product keys at hand. Note that this reveals the right most part of the ................-3V66T

+1 on Magical Jelly Bean Keyfinder. I've used it many times to help friends recover their Windows product keys. Link

Windows Product Key Finder Pro is Freeware and works great. As it installs, be sure to uncheck the toolbars and updating program it offers.

Windows Product Key Finder Professional is a software utility which allows you to find and recover your CD Product key for Windows or Office programs for the purposes of reinstalling / repairing your Windows or Office setup. Recover your lost key or serial for Windows 7, Vista, XP, 2000, 2003, 2008. Office 2007, 2010, 2003, XP.