4PCs: Vista1 Vista2 Win8 Win10 using Workgroup Network:
The older PCs still behave normally, can see each other and Win10.
The Win10 PC has two problems: 1) It can't see Win8 (sees only the other 3). 2) Win10 Net View in command prompt shows no PCs, just a message about "system error 1231" and "see windows help". An older similar thread mentioned the net view problem may be related to the computer browser service, but it's running.

This is my first attempt to install Windows 10, so I have no previous installs to refer to or compare. In the past 15 years of home networking I think this is the first time a PC was totally invisible to only one other PC. It looks like the problem is Windows 10, considering the strange result from the net view command.