|Or it could even be as simple as a cable gone bad. Do you have another one that you can try? It could just possibly be your router/switch, but that's the least likely cause. Anyway, network cards and cables are not very expensive so it's easily corrected.|
You might just want to try reinstalling the driver for your network card before spending money; otherwise I would just buy a new network card and cable (it's always useful to have a spare cable in any case), disable your built-in NIC in the BIOS (if it is built-in), and install the new card and drivers.