I have an older computer that had a dead motherboard (Socket 939). I found a used replacement for free (made by PC Partners). I found out that the onboard LAN doesn't work so I picked up a 10/100 NIC card. Windows XP.
I installed the proper Realtek drivers for the card and it is fully recognized by the PC, but cannot connect to the internet. The card is connected directly to a cable modem (no router). What it basically tells me is that it cannot acquire an IP address. On one or two occasions after rebooting, it actually did acquire an ip address and showed as "connected" with IP and gateway info, but I still couldn't access the internet. After I rebooted it could not acquire an IP address again. It is set to automatically obtain an IP address and DNS.
I've now tried two different NIC cards with the same results. I've also tried another computer and that one was able to connect to the internet through the modem, so the modem/cable service is good.
The BIOS in this crap motherboard is limited, and I could find no option to disable the onboard LAN, nor enable a PCI LAN card.