Attempting to connect to an administrative share will bring up a login box if the computer can be found. This was suggested as a troubleshooting step. NO drive mappings required. If you get a "could not connect" you have a hardware or NetBIOS issue. Now try the same thing, but substitute the machine's IP address for "machinename". (I assume you did substitute the actual machine name for "machinename"?) If you get a login box, your problem is definitely a NetBIOS issue.
Try installing NetBEUI.
http://support.microsoft.com/default...301041&sd=tech