Lost Network Connection

I have a small network of three pcs running WinXP.

PC-A and PC-B access files on PC-C.

On ocassion, PC-C will access files on PC-A and PC-B. PC-C will always
access the printer on PC-B.

All of a sudden, I think, a few weeks ago, there was a FireFox update
and when the computer rebooted, PC-C could no longer access PC-A or
PC-B: neither drives or printers.

PC-A and PC-B can still access drives on PC-C.

I have tried many things to try to correct the problem, including a
complete re-install of Windows. I still have the same problem. I would
like to think it is hardware, but how could others access PC-C and
PC-C can access the internet through the same ethernet connection?

Any help, Please!

TIA

Joe