PC can't even see itself in MSHOME

I have 3 computers connected via a wireless router. All can connect to
the Internet fine. All can also ping each other via their local network
address. The problem I have is with the home network I've set up to enable
file and printer sharing. All 3 computers are running XP and I ran the
included Network Setup on each.
Here's the puzzle: 2 of the computers can see each other under MSHOME,
but the 3rd computer is not visible to the others, cannont see the others,
and cannot even see itself under MSHOME.
Even more oddly, on this 3rd computer, when I go to the Network
Connections window (under Control Panel) and right click on the Wireless
Network and go to Properties, I see that QoS Packet Scheduler is checked and
so is Internet Protocal (TCP/IP), but Client for Microsoft Networks is not
check. Also File and Printer Sharing for Microsoft Networks is not checked.
When I try to check those two, I get this paradoxical message upon clicking
OK:

Your current selection will also disable the following components:
Client for Microsoft Networks
File and Printer Sharing for Microsoft Networks
Are you sure you want to disable these components?

How can checking on two unchecked items disable them????

At this point, I have a choice of clicking Yes or No, but it doesn't matter.
Whichever I click, when I return to the properties screen, I see that Client
for Microsoft Networks and File and Printer Sharing for Microsoft Networks
are still not unchecked.

Does anybody have any suggestion on how to get this 3rd computer connected
to the network correctly? (FYI: At one point, all 3 computers did operate
on this network correctly. Somewhere along the way, something I did on this
3rd computer must have messed up some setting, but I don't have a clue as to
what it was.)