client loses connection to network

Hello
New to this so please bear with me.
I have a small lan , one file server and 7 clients.
The server is running w2k and the machines XP service pack 2.
Have a router and a switch to connect all.
The problem I am having is recent I just do not know how recent.
All the machines work fine all day but after about six or seven hours they become very slow accessing files from server.
Also if a machine is left running and logged in to domain all night in the morning cannot access files on server. I can ping the server from said machine but that’s all.
Just in case running on server Norton antivirus corporateand tapeware backup.
Thanks in advance. Any suggestions are greatly appreciated