Hi Andre,
Thanks for that. I did the removal reinstall on both systems, lo and behold, they could file share again with the firewall on. But.. the next day, after rebooting, computer a fails again to see the file share on computer b (strangely, the b can see a just fine). Turn the firewall off on b, and it's fine again. It worked for two days and I made no changes to the systems or network in the interim.
I think -something- is still hosed in Avast w/Win 8.1.
Not sure what, though. Looking forward to try the next update.
-Daniel