iMac won't maintain network connection to pc

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.

The iMac is a mid-2011 I7 running the latest OS X 10.8 Mountain Lion. All PC's are running Windows 7 Ultimate. The iMac has no problems with internet connection.

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.

Since installing Mountain Lion I am unable to connect to my PCs. I had no trouble with Lion. I can still connect to my PC server through remote desktop, but not directly to my drives on the pcs.

It appears that many of my problems were due to routing through a 2WIRE DSL/router box. With my current ISP, I have no choice but to use it for internet access. However, tonight I stuck a D-Link gigabit router in front of it, and networked everything through that, instead, with a single line running to it from the 2WIRE. All computers connect/mount without issue, now, and my transfer speed went from 0.4 Mbps to 45 Mbps. That makes me a lot happier. I had been reading bad things about the 2WIRE boxes, and when I talked to the techie at the local Apple dealer today, he also said the 2WIRE was likely the problem.

Bob_Stan said:

Since installing Mountain Lion I am unable to connect to my PCs. I had no trouble with Lion. I can still connect to my PC server through remote desktop, but not directly to my drives on the pcs.

Click to expand...

To extend the story a bit... I got to the point where I couldn't even connect to the shares on the one Windows PC. The iMac could see it on the network, and I could vpn onto it and operate it, but it would just keep asking for the password (and rejecting it) when I tried to connect to an SMB share. I tried everything imaginable, but had no luck until yesterday. I was about ready to give up, when I noticed that the date on the Windows pc was wrong. The date on the pc was Sep 2, when it was actually Sep 4. I bumped it up to the current date, then tried connecting to the SMB shares again, not expecting that to make a difference. However, it solved all my problems. I could connect to all shares immediately, and the connections haven't dropped since. Go figure. Anyways, the chances that your problem has the same solution are pretty thin, but I thought I would mention it.

Just to follow this up... I have had no connection or disconnection problems since correcting the current date on the Windows PC. Weird, but it's working great now.