XP Client Slow over the Network

Hi:
I have a setup of WIN2K Domain Controller and around 50 XP Clients. I have a WIN2K File server. When clients access the Documents over the network, it takes forever to access those documents. When they restart their comptuer they can access the files quickly but after a while (like 2-3 hours) it again takes forever to access files. Even the logon on client computer takes atleast more than 3 minutes. This is very frustratring for everybody.
Any help would be appreciated.
Thanks.
Rajiv Sharma