Thank you. Following the suggestions at
> http://www.ezlan.net/sharing.html
I went to Safe Mode and allowed all access in the Security tab that
appeared. Now the computer shows up in the Folders pane of the old computer
in the workgroup under Network Neighborhood\Entire Network\X (where X is the
name of the workgroup) but in the details pane of that same Windows explorer
I see only the old computer and the other Win XP, but not the problematic
computer.
Firewall was already shut off.
I can still access the problematic computer by typing its UNC (\\Fish) in
the Explorer address and I see it all, and now also by double-clicking its
name on the left Folders pane, but when I go Up I'm in the Workgroup and I
can't see it.