I had a 2-PC network.  Call the 2 machines "server" and client.  (Neither server NOR client accesses the Internet - it is a 'closed' network).

What we did:  We replaced the server with a much faster model.  EVERYTHING works!  server can see client, client can see server, the PCs can exchange files.  Applications with data on server work.  The only problem???   S  L  O  W  !   (especially when copying files).

What did I do? 
    1.  I set up the new server (made it part of the workgroup)
    2.  I pulled the CAT-5 cable out of the back of the old server and plugged it into the back of the new server.

And, as I said, everything works but   S  L  O  W.
The server itself is lightening fast but it took me like 60 minutes to copy a file (from the server to the client) that normally takes about 4 minutes.

One person suggested that the problem may be the "duplex settings" of the network cards involved OR the cabling.  If the cabling were "bad", then it is bad in a way that did not "offend" the old server but DOES "offend" the new server.  I suppose that is possible.

Thank you.