Can I backup different windows operating systems, e.g. Windows XP Home, Windows XP Pro and Windows 7, on one external hard drive using the Windows Backup program and then retrieve them from the drive if needed?
According to me, Yes.
It will create a*.bkf file for each OS.
and you will restore that bkf file respectively.
So it does not care how many other bkf files are stored and which OS it contains.
Carefully name them as xp, 7, vista.
so it will be easy to restore them.
possible better to give different names for each one