|Assuming the NIC is okay and only needs a driver, it should show up in Device Manager as an unknown device. The problem with this is that you need to know what NIC it is, in order to download the driver.|
If it's a NIC card (e.g. PCI), you could open the case and note any numbers and/or text on the card. Do a google search for these numbers/text.
Or, you could try downloading and installing Aida. It should be able to identify the NIC.
It must be a very old (or very new) NIC if Windows XP doesn't have a driver for it. Try rebooting the machine and see if Windows detects the NIC. If not, it may have gone south.
Please let us know if you found someone's advice to be helpful.