Vista Workstation missing

I have a Home Network with wired 2 - desktops and wifi 2 - laptops and a
network printer. One desktop is running Vista Ultimate and the rest have XP
Pro.

On the Vista desktop I got the discovery on; file and printer sharing on.
When I check the network screen on the Vista machine all other workstations
on the network are visible and shared directories accessible including the
printer. The same thing with the XP workstatins except that the Vista machine
is not visible on the Network screen of the XP workstations. However, if I go
to a XP machine and map a shared directory on the Vista machine it is
successfuk and be able to access the directory on the Vista machine.

Is there a step I missed that the Vista machine is not visible on the XP
machines. Your input and help is very much appreciated.

Mike