I stupidly upgraded from Win7 Home Premium last munth to Win7 Ultimate, Now I’m getting the “your copy of Windows is counterfeit” is there a way i can go back to my genuine Win7 Home Premium?

I have the windows 7 disk and I have a windows vista disk. My win. 7 is actin up. I want to go back to vista,but it wont let me downgrade. How can I revert back to my normal windows before i upgraded?

I bought a laptop with windows 7 installed and i want windows xp on it instead, so i’d like to know how to do that?
How to downgrade?

I want to downgrade from windows 7 to xp, can anyone help or tell me if formatting will make it work?
Thanks