Do you get any errors on shutdown? What exactly happens when you try to power down? What command do you use to power down?
NIC--do you have any lights on the NIC? What happens when you reboot--does it light up, or nothing at all? Have you tried to reseat the NIC to make sure it's securely fit into place? Have you tried the NIC in a different slot? Do you get any errors when you type: "ifconfig eth0 up" (without quotes and using the correct assignment for the NIC in case it's not eth0) in command line?
The more information we have, the better we can help. Thanks.