iMac won't maintain network connection to pc

[Lastmboy]

Just got my first iMac. I have it up and running and on a network with two Windows PCs. The iMac immediately spotted both PCs on the network. For the first, I clicked on "Connect As..." and entered the credentials. It works fine. However, the other one wouldn't connect. It says "Connection Failed". If I click on the "Connect As..." button, it says "There was a problem connecting to the server {...}". However, it has not prompted me for any credentials, and won't allow me to input them anywhere. I re-booted both the iMac and the pc I was trying to connect to. It showed up and let me connect. I copied a few files across with success, then suddenly it dropped it again, and just says connection failed. The pc has been on this network for years and has been fully accessible from all other PC's and devices. I'm actually using it as a file server.

Why will it not let me enter login credentials?
How did it connect once, without me actually entering the credentials?
Is there anywhere else I can go to set the login credentials for this pc?
Is there anything I can do to get it to hold the connection?

Thanks. I'm stumped, and can't seem to find documented solution.

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I should have mentioned that the iMac is a mid-2011 I7 running OS X 10.8 Mountain Lion. The PCs on the network are all running Windows 7 Ultimate 64-bit. Also, the iMac has not problems with the internet connection, and generally can see the other pc on the network, although it has disappeared a couple of times.

 

[c5lm]

Networking Problems

I too am having connectivity problems networking between an iMac (Mid 2011) and a PC on a peer to peer. This issue just surfaced after installing Mountain Lion. I have disabled firewalls which does not resolve the problem.
I can ping from the iMac to the PC and from the PC to the iMac.
Any help would be appreciated.

Thanks.

 

[Lastmboy]

I too am having connectivity problems networking between an iMac (Mid 2011) and a PC on a peer to peer. This issue just surfaced after installing Mountain Lion. I have disabled firewalls which does not resolve the problem.
I can ping from the iMac to the PC and from the PC to the iMac.
Any help would be appreciated.

Thanks.

This may not help with your overall problem, but something that I discovered with the one pc I couldn't connect to... when I compared it to the pc that worked fine, I discovered one difference - in Advanced Sharing Settings, I needed to select "Turn on password protected sharing". Doing this, then un-mounting and re-mounting it on the Mac seemed to solve most of my issues.

 

[c5lm]

Thanks for the tip but it didn't help at all. It made networking more restrictive.

 

[NewbieCanada]

Something I find usually helps:

System Preferences, Sharing, File Sharing, Options:

Uncheck "Share files and folders using SMB (Windows)"

Check it again.

I find I need to do this after every reboot of either the Mac or the PC, under both Lion and Mountain Lion (running Windows 7 on the PC)

 

[c5lm]

Thanks for the tip RE: checking SMB in file share.