haven't used it that much, but get your hands on "whatsup gold"
and... are only the PC's having this error message? are they running any apps different then other computers? do you have old switches, hubs?
another thing, windows server comes with a utility called "network monitor" you can capture raw network traffic. Not sure if you are going to be familiar with what you see. But if you really can not find any documentation on your problem than i would look into learning that. Do you get a stop error (blue screen)? or a error message? what doies it exactly say? is there any erro number associated with it? did you go through event veiwer on the machine having problem as well as the server?