Shutdown then disconnect the RJ-45 cable from the MOBO, and restart. Open the Device Manager, click [+] Network Adapters - does Realtek 8111C/.... show-up? If not click the Scan for hardware or Options ? Scan for Hardware.
If it wasn't there before then you've got a bad cable.
Before going further, is the Realtek 8111C/... listed in the Device Manager?
Also, try -> http://support.microsoft.com/kb/299357