Did you install the nForce Drivers? You need them for all motherboard devices. Do not rely on the generic Windows drivers.
Try an PCI NIC just to test. If it works with that then you know the problem is with the onboard NIC. You may need updated drivers or may just need to uninstall and re-install the driver.
Did you install the NVidia firewall? A lot of people have had problems with it. Try uninstalling it.
Hope this helps...