Dual boot - drive letters.

We have 2 machines with XP and Vista dual boot.
On the one machine the OS I boot into always shows itself as being on C:\
and the other OS on D:\. In other words: When I boot into XP then XP is on C
drive and when I boot into Vista then Vista is on C drive.
On the second machine XP is ALWAYS on C:\ and Vista always on D:\
I like the idea of my OS always being on C:\ drive as that's what I'm used to.
Is there a way to make the second machnine re-assign the drive letters
depending on which one I boot into without re-installing Vista??