Vista - Cannot Connect through LAN

I recently built a PC that is using a Gigabyte motherboard and Windows Vista Home Premium OS. I am directly connected to a modem, but the computer is only detecting a wireless signal, so I'm able to connect wirelessly but not locally. The cable light on the modem is receiving activity, so I'm not sure what's causing the problem. What is most likely the issue here? I'm really hoping that the ethernet card isn't bad, but if it is, what are my options other than replacing the motherboard?

Make sure the lan isn't disabled, I know some wireless connections will disable the lan automatically so they take priority of the connection. Also if the wifi and lan are both onboard, make sure both are enabled in the bios.

Go to Control Panel > Network & Sharing Center > Manage Network Connections.

1. Is there an entry for Local Area Connection? If not, you probably need to install drivers for your ethernet NIC. Or, as already mentioned, enable the ethernet port in your system BIOS.

2. If Local Area Connection exists, what is its status?

That's a starting place... let's go from there.