Since no one else has made an attempt at answering, I will give it a try.
Can I assume the network adapter is integrated with the motherboard? I have seen people add a PCI network adapter to their computer when one already existed. If that's the case, you need drivers for that card or plug your CAT-5 cable into the motherboard port.
If you are plugging into the motherboard port and it is not working, I would add a network card to a PCI slot and load the drivers. It's not like the card is a huge capital outlay and could easily fix your problem.
I hope all works out for you. Have a nice weekend.