Dual Boot

I’ve read about everything on the site about dual-booting but can’t find anything that answers my situation.
I purchased a new machine with Vista installed. No problem there, works great. I then imaged (cloned) my old XP system onto a removable hard drive and connected that drive to the Vista machine. I can read all the folders, files, etc. but have not yet figured out how to configure the dual-boot. Since I didn’t really install XP over Vista, Vista doesn’t really know it’s there and, therein, doesn’t provide the option in the dual boot menu.
Any suggestions or guidance would be appreciated.
John.