Just to clarify, this computer was able to connect to the internet before, right?
Secondly, to go to the BIOS, upon bootup, it should tell you a key to press to get to the BIOS or "Setup". Common keys are F1, Esc and Del (delete). The screen is at the very beginning, sometimes during the RAM count. Browse through the settings, and look for your setting.
Thirdly, it's possible that XP doesn't have a driver for the network device. There should be a section called unknown devices in the device manager, and take a look there and post back what it says.
Lastly, is this an onboard network interface, or is it an external card?
FYI: The Loopback adapter has nothing to do with your network card. It's pretty much like directing everything to yourself, even outward connections. Hence the name, loopback.
Post back the results.