Maybe you guys still wanna back to use Windows 7, so you need a product key to activate.
You can find some fine working and genuine Windows 7 product keys all editions on this site.
I'd love to share some links. :)
You can also get some more from here: Windows/Office product keys.
To activate by using a direct product key, here are the steps in case you forgot.
1. Open Windows Activation by clicking the Start button , right-clicking Computer, clicking Properties, and then clicking Activate Windows now.
2. If Windows detects an Internet connection, click Activate Windows online now.
If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
3. Type your Windows 7 product key when prompted, click Next, and then follow the instructions.
Hope that you can get what you want from my experience. :)