Bewildering Network Problem

I just installed an EVGA 680i A1 LT motherboard with no problems (it has an on-board network adapter). The system booted up all the way, I installed the board drivers and everything worked great except the internet.

The internet works fine on the other computers on the network, whether connected to the router or directly to the modem. I tried resetting both router and modem but that didn't work. I tried static and dynamic IP. I even reinstalled WinXP32, because the manual said the OS needed reinstalling. Now, with a non-updatable OS, my hands are tied.

What's going on? Did I get a defective board?

Also, at the start it was working but very slowly. It would load half the "Google" image in 2mins, then stop working. Now it plain doesn't work. And I can't access router or modem with it.