Networked computers disappear

Where I work we have a small network with a Windows Server 2003 machine,
four Windows XP Pro machines, and my recently built Vista Business system.
We really do not use the server to it's fullest potential seeing as we only
really use it for sharing a few folders. We share printers from the other
computers but everything works fine.....except on the Vista system.

Not to say it does not work, it does....just in an unpredictable manner. I
can ping every computer on the network, both via their IP addresses and
their computer names. I have mapped drives leading to the server and they
work. I can even open the RUN dialog box and type in the path to a computer
(e.g.. \\server) and the shared resources of that system are displayed.
And of course I can print to the share printers. It all WORKS, but when I
open the Network section of the Windows Explorer sometimes all the networked
computers are there and sometimes only my computer is displayed. I am not
willing to say that this is exclusive to the Vista machine, just that this
is the system I use and experience it on.

As I said, everything still works, it is just VERY puzzling and annoying and
I would appreciate any idea.