PC automatically goto out fo network

Hi,
I have a here one problem in windows 98 PC
When I connect to domain ( windows 2000 server).
it is connect properly and work LAN is proparly
but after 1-2 hours PC automatically disconnect from network so I can not able to access any computer but still ping command is working, when I pinging it is give me reply properly.

Plz give a solution.

regards,
Narayan Prajapati