The shares on the one XP Pro machine on a LAN don't show up in the
Network Neighborhood under the workgroup, for some unknown reason. The
computer name is not duplicated anywhere else on the network and the
workgroup name is the same as the others on the network; I can't
identify any service not running on the affected machine that is running
on the others. The most useful observations may be these:

* A dir \\IP_OF_AFFECTED_MACHINE\share\ produces the same result on any
given machine as a dir of \\NAME_OF_AFFECTED_MACHINE\share\ so it's not
a name resolution problem.
* A ping IP_OF_ANY_MACHINE_ON_NETWORK works, with typical response times
under 10ms. The affected machine included. So the network's physical
integrity isn't at issue, nor is the affected machine not participating
due to a misconfiguration low in the protocol stack. All the machines
can see the Internet through the gateway, also.
* Doing a dir of a share of the affected machine on the affected
machine, using \\NAME\share\ rather than C:\share_directory\, produces
"The network path was not found" despite the above.
* Interestingly, doing the same dir from another machine produces "Logon
failure: unknown user name or bad password."

All of the above suggests it's a permissions issue, though the machine
not being visible in n'hood AT ALL is odd. Checking the sharing tab of
the properties dialog for the share on each machine shows that the
affected machine's tab looks very different, probably because it's
running Pro instead of Home. The tabs on the others just have a basic
checkbox for share this directory and allow others to read only vs.
change stuff; the tab on the Pro machine has a concept of distinct users
with separate permissions. However, the Pro's share has "Everyone" able
to "read" the share. The same directory seems to exist as two
simultaneous shares, however, one of which I was told existed when the
machine was new, created by the OEM. It has the comment "Default share"
and the permissions aren't accessible -- it pops up a box saying the
share was created for administrative purposes (by the OEM?) and
permissions cannot be changed. Apparently, they cannot even be *viewed*.

A possibility is that the two shares conflict in some way, but I'm loath
to remove the pre-existing share -- if it exists for "administrative
purposes" it may mean that something in XP Pro requires this share to
exist to work properly, and eliminating it might therefore cause problems.

--
There's only four things you can be certain of: taxes, change, spam, and
death.