I'm having the same problem with my new NForce2 system. Once I run the network wizard it all comes back, but I find the prospect of running this everytime I boot not very appealing. I am also experiencing an abnormally long delay when I shut down. I just did a clean install on this box with all the windows update and latest drivers. My internet connection is DSL PPoE connected through a D-Link DI-604 router.
One weird thing is that the router's DHCP seems to be detecting 2 different MAC address from my computer. My BIOS has some MAC address settings, I will try setting it to one fixed address. If this doesn't solve the problem, I think I'll just put in my trusty old NIC and disable the onboard LAN.