Installing the XP Hdd in the new computer as the sole disk drive works fine. The operating system boots and programs execute normally. And of course, installing the Vista disk alone works also. It would seem that with a couple of carefully placed key strokes, placement of the necessary files in the correct folders and this should just work. Pot luck or any other kind of luck should have nothing to do with it... lol
I'll give it another go this weekend and will keep you posted.

If you have both of the OS's working independent of each other this guide
http://www.pronetworks.org/index.php/guides/post/unplugged_xp_drive_for_vista_setup_cant_dual_boot_to_xp/

Or his guide either one should work
http://www.pronetworks.org/index.php/guides/post/dual_boot_trouble_shooting_guide_vista_running/