Hiding each OS from each other
When setting up XP and Windows 7 as duel boot (heh), I found it desirable to hide the OSs from each other. Each would see it's own small partition as "C:", not see the other OS's "C:" at all, and share access to an extended partition containing logical volumes D: to F: and the optical and USB drives beyond that.
This prevents the engine behind "previous versions" and System Restore from screwing with each other's SVI (something to test, please!) and other hassles. I used a free boot manager to select between the OSs, hiding what was not in use.