NIC Disables itself!

Maybe someone here can give me a hint on my current problem. I have been setting up a LAN at a business and on one particular system, no matter what I do it will not network with the other PC's. The system is a 1Ghz Windows 2000 PC, cleanly formatted. The network card I am trying to use is a 3com 950c. This is what happens, the network card installs perfectly fine, being recognized by the OS and self-installing all the drivers. However as soon as I plug the Cat5 cable in, it disables itself. Atfirst I thought maybe it was a defective card, but I have tried 2 other 3Com cards and 2 other NICs with the same outcome. Then I thought perhaps it was the Cat5 cable, but I have tried 2 cables and tested those cables on another network. I thought it might be conflicting with the US Robotics 56K sportser modem, but I have disabled it and I still get the same outcome. I have checked for IRQ conflicts and come up with nothing. As long as the cable is unplugged I can view and change the properties for the NIC without any problems, but once again as soon as I plug in the network cable it disables itself. I have even purchased a Belkin USB ethernet adapter to test to see if that would help, but once installed the machine would reboot to a BSOD. I have tested the NIC in the system with onboard sound disabled, LTP1 disabled, USB disabled, and even tried a new video card. There is no onboard NIC so I know that is not it and I even tried a different modem in the machine to see if that could be it. Other than that the only odd thing about the machine is that I was unable to setup the LAN when the machine had Win98 on it, but I figured that it was because of a bad install or corrupted HDD (which I also swapped out). So I am out of ideas, anyone got anything else I can try before these people think I am trying to rip them off by inflating my hours?