Conventional wisdom states that in order to dual boot two versions of Windows, the oldest version must be installed first otherwise only the last version installed will be bootable.
Microsoft exacerbated this by totally revamping the way boot settings are stored and edited in Windows Vista. Instead of an easily edited text file (boot.ini) users must edit the boot settings from a command line tool. And, of course, that tool doesn't work in Windows XP.
So what are people to do if they wish to set up a dual boot with Vista and XP if their computer already has Vista installed? One option is to wipe the drive and do everything from scratch. This, however, may not be a viable solution with OEM versions of Windows Vista which may not give the user the necessary options while installing (or may wipe out the drive while 'repairing' the OEM installation).
Enter a nifty bit of programming called VistaBootPro
Among other things, it allows users to edit the Vista Boot configuration from within XP.
So, simply boot into Vista, repartition your drive, reboot with your XP disk, install XP, boot into XP, and run the VistaBootPro program.
I threw together a short tutorial
for a client of mine, and I'd like to share it with the internet people.