First, visit the computer manufacturer's website, type in the model number and check for ethernet card drivers.
If there are none there, then you'll need to find the ethernet card's manufacturer and visit their site to find a driver. To find the ethernet card's manufacturer, you'll need to check in Device Manager. On Windows XP, RIGHT click on "My Computer", choose "Properties", then select the "Hardware" tab, then the "Device Manger" button. When that opens, click on the + sign next to "Network Adapters" and look for the ethernet device listed. You should have two listings. One for a wireless adapter and one for the "Fast Ethernet". RIGHT click on the ethernet device listing, choose "Properties", then click on the "Driver" tab. When that opens, write down the exact DEvice Name, the "Manufacturer/Provider" name, the driver version, and the driver date.
With that information in hand, you should be able to visit the manufacturer's website and download updated drivers, then install them.
Hope this helps.