Have a look under Device Manager [Control Panel -> (Hardware and Sound) -> Device Manager]. There should be a tab for Network Adapters, and under there you should see any network adapters your computer can see. If the name gives away the type or name of adapter, try downloading it from the respective website (e.g. Intel 82579V or Realtek PCIe GBE Family).
I am however assuming you are running windows vista, so perhaps a bit more details about your system and current hardware would help diagnose the problem.