How do you get back to windows vista when I have already installed Windows 7 and it says that my activation key is inactive?

Different version of Windows use a different activation key. When you did the right thing by yourself in upgrading to Windows 7 that disabled you vista key. Even Microsoft has described Windows 7 as Vista with the bugs squashed. Most people who can have dumped Vista and enjoyed the change up to Windows 7. Contact Microsoft on how you can downgrade your operating system