NT server hangs up

I have a server with NT4 as OS. 12 win98 workstations are connected to server and they are using server as file server only. A dos programme is running on all the workstations which are using a folder on the server. Server hangs up after couple of hours and reboot is required. If Workstations are restarted, it gives following message ” No domain found to log on….”.
I will apreciate all kind of help.