First..thanks for all the replies....
From my own trials, it appears (at least for my set-up) that the solution is indeed, tied to user/password combinations....initially, none of my PCs had pws and I could browse Windows 7<=>vista, but not -->xp...could also browse xp to both Windows 7 and vista. After creating the same user/pwd combo on all three PCs and making them the active user on each PC, I could browse every which way with no problems...then, I started messing around by removing pwds and trying to browse around....
Observations(I think)...
I couldn't find circumstances (user/pwd combos) which kept win 7 and vista from browsing with each other...
xp requires a pwd for its active user in order to browse other OSs...other OSs require at least one user (who does not have to be the active user) who has a pwd...
assuming this to be the case,...
xp will browse with either of the other two OSs, if active user/pwd the same as xp....
xp will browse with other OSs if activer user does not have a pwd, but a non-active user does exist who has a pwd and that combo is known to xp user...xp will prompt for user/pwd...
if other OS active user does not have pwd, but wants to browse xp, user will be prompted for an xp user/pwd....
if I had a standard hold harmless, I'd put it here(

), but these observations worked for me...