Network cable is disconnected

I have a very strange problem with an RTL8139 NIC.
It is showing up in Device manager as being installed correctly and seemingly working.
Problem is, it will not connect to the local network, with a “network cable is disconnected” mesage.
I have tried uninstalling an reinstalling and updated the driver, with no success. I have also tried every speed in the configuration to make it work, still no dice.
This is an onboard nic, on Windows XP.
Any ideas out there?