I have an ABIT IT7 mainboard with an onboard Realtek 10/100 NIC. The system runs Windows CP SP1. Although the card is enabled it says that the network cable is unplugged which it isn't. I am attempting to connect two PCs and I am using a cross-over cable. Has anyone come across this?
First make sure you have TCP/IP installed and bound to the interface. To test this, ping the IP number of PC from the other, make each PC ping it's own IP and make each PC ping 127.0.0.1 This is a loopback IP and tests the workability of your TCP/IP setup. Assuming the latter two tests worked, your fine and it's a PC to PC issue.
I had the exact same problem w/ my LAN using Realtek. Never solved the problem, just changed to a different NIC brand (3COM) and it worked w/ a hub. Realtek cards can have major problems w/ ALi or AMI chipsets I heard somewhere, so try updating your BIOS if that helps at all.