Losing Internet connection but maintaining LAN connection

Heres the situation:
I have 6 computers on my network(all xp pro), running server 2003. On one of the pc’s I have recently been having trouble losing internet connectivity. I am still able to access local files on the network and run software which accesses file on the server. This seems to happen every morning when my client comes in– he is able to restart the computer and regain internet connectivity until it happens again the following morning.
We are using Cisco equipment.
Any support would be greatly appreciated.