Darndest thing! I finally gave in an reloaded Word Perfect from the original disc. I installed it in D drive ready for the Dual boot W7 in D, to make use of it.
I fully expected to have to install it again in C drive, to get Vista's copy working again.
To my astonishment, either boot is making use of one copy. Very desirable space-wise, but how is it doing it, and are there any traps I'm about to fall in?