XP permissions/locations

I have a very small network – 3 machines, 2 x NT4(SP6), 1 x XPpro(SP2), XP machine connected via wireless, NT machine via cable – but all through the same router and all members of the same workgroup. All network IDs are correct and all machines are visible to each other in ‘network neighbourhood’. Other normal tests for network connectivity are positive. I cannot set permissions (on the XP machine) for the NT machines to login to specific directories. If I try
(folder)> Properties > Security > Add > Locations
… all I see is the local (XP) machine. All three machines have the same usernames/passwords ( although two of the latter are blank).
Sorry if this is particularly trivial, but all suggestions would be welcome.