You can install a NIC card quickly, easily, and cheaply... $14 to $20. If the device manager in System gives you trouble, disable the old Network interface in the device manager. Then install drives from the new NIC in the PCI slot. Often in WXP, Windows will automatically install the new NIC card. If not, the drivers will. Just go back to System, and look in the device manager to assure that it is installed properly and is working.
One more thing. Many times it is not the computer, but the cable. CAT 5 and CAT-6 internet cables have an extremely high failure rate at the plug, so you might also want to test or replace the cable with the best one you can afford... not the most expensive, bu tthe best.