Network Problems with Server 2003

Hi,

I have a big problem with the Server 2003 Standard Edition. It’s a Domain Controller and it has 5 normal users(no domain users)with shared internet connection.
When connecting the server, internet goes down.
But locally it’s fine.
When deconnect the server, everything is ok again.
Checked IP config. Checked cables, pinging locally always. But outside is a problem.

I also connected this server with the router (act as a switch) and I get link. But when connecting it to the Modem, the server is not getting any link. Other users are getting link as well.

Maybe NIC defective?
Please help. I’m a long time busy to fix this out.