There are several things you can do here. I'd recommend trying the following steps:
■ Windows just might not be playing nice at all. You could try creating a
Ubuntu live CD to see if the ethernet port is still not working. If it works under Ubuntu then I think it's a driver issue.
■ If the port doesn't work under Ubuntu I think you've got a hardware problem. On the
ASUS website they do seem to have a 0601 bios update for "Enhance LAN stability". Perhaps you could try flashing to this bios version?
■ If a bios update doesn't solve it you have two options. You can either try returning your board for something else or you'll need to buy an add on network card. It's annoying but they aren't too expensive these days.