Suddenly Lost Full LAN Access Of Computers

[NewW7User]

I have three computers on my LAN. I normally access all files on all computers from any of the computers. Suddenly today I can't access anything on Computer C from Computer A nor from Computer B. In Map Network Drive on Computer A and on Computer B it sees Computer C. But when I click on the '+' it says Computer C isn't there. I can access everything on Compute A and everything on Computer B FROM Computer C. Just not the reverse.

I didn't change anything today but apparently something got changed somewhere. Can anyone suggest what to do and/or where to look?

UPDATE:
I just noticed that Avast Premium Security got updated today. I turned off the Avast shields and now Computer C CAN BE accessed. Something in this Avast update apparently is the problem. I'll open a ticket with Avast but I'm afraid they won't do anything about it because it's running on Win 7.

 

[SIW2]

Is there something wrong with using mse ? works fine here
https://www.microsoft.com/en-gb/download/details.aspx?id=5201