Want to go back to Vista Home Edition from Windows 7

My friends laptop has Windows 7 Ultimate installed on and she doesn't seem to remember doing it, possibly one of the teenagers not exactly sure how that happened but I want to put the operating system back to Vista for obvious reasons...can't use any of the software installed under vista without reinstalling. It seems to have the folder called Windows.old available but I am unsure of what steps to take to get the laptop back to Vista. Any suggestions?