Connection Problems with Server 2003

Hello Major Geeks
Having a major problem with a Server 2003 network. We have about 15 workstations connected to a single Dell Poweredge server running Server 2003. Been running fine for years. We have 2 major groups of workstations connected. About 9 XP Pro Dells and 7 old Windows 98 machines. This is an industrial environment -- foundry. Yesterday out of the blue, two thirds of the clients won't connect any more. Not related to hardware, Everybody still sees the web just fine, but only a select few of the XP Pro machines will still see the Server. I can ping the server from the downed stations, but they will NOT connect -- Can't map to the shares. None of the Windows 98 machines will connect and we have one Windows 2000 machine in the foundry that is still connected. We set all of this up and have dabbled with Group Policy and we have a domain set up. We are not MCSE's though. Any help would be much appreciated.