List of OS to install at startup

[ihaveaproblem]

I beleive this is possible but I cant think how to do it.

On a external hard drive lets say i extract the windows vista files to it. After changing the boot order etc in the bios the computer can now boot from the files on the device and install Vista.

Now lets say i extract the Vista files and windows 7 files to the hard drive. I guess the computer would get confused now and wouldnt know which to boot from?

Is there a way I can make a list appear at startup like when you dual boot which lists all the possible OS installations on the hard drive and let me select when one to load?