Slow closing and login

A win2000 server and workstations, workstations are vary slow to shut down and login. A laptop takes 20 to 30 min. to login after its in it works fine. Running 10/100 nics, all MS compliant. Replaced each nic to see if one is bad, ran lan check to see where the slow down is it showed all running at speed. Checked nds and dhcp both are ok. Network cables and connections are ok. Server is a white box, 40gig-30gig, 512mem, 2gighrtz-print server 40gig, 512mem- workstations 1.5gighrtz, 196mem.