How do I reinstall a old copy of windows vista?

Hi, I recently installed windows 8 on my compaq presario cq50 and it turned out to not be genuine. I want to reset the laptop to factory settings. I was wondering if I have to back it up to do this because there is nothing on it besides the system and browser. I dont know if I have the vista home premium disk that it came with in 2007 (I was six back then) but it has a product key.

Any solutions?

Thanks, Ted