File and Print Sharing and ICS stopped working

I've had a small network in my home for two years fairly trouble free. All
boxes are Win98SE with NICs connected to a hub with CAT5E. One has a modem
and ICS. The ICS box dials up the ISP and the others simply launch IE. All
could share files and printers also.

Last week my daughter said she could not get Instant Messanger to connect. I
checked and her machine could not see two of the four. And the two she
couldn't get to could only see each other. I tried changing workgroup names
and computer names but the same pair can only see each other in network
neighborhood. So now it's like I have two different networks each with two
workstations. I know this can happen if you use different workgroup names
but I was careful when I typed - and no uppercase or trailing spaces or
special characters. In fact I went back to the default name "WORKGROUP".
Same issue.

It isn't a physical connection problem because I swapped the machines around
to different jacks the symptom hasn't changed.

Are ther known problems with Win98 and networking that I can investigate?

Thanks in advance.