Is the pci card stand-alone, eg: added to motherboard by you or was it installed by Gigabyte when you bought the pc. It's still very difficult to help you until we know that information.
However, my add-on RTL8139 pci card was plug and play in Windows XP. Microsoft included native support for it. I would think yours would respond identical unless you have a bus problem. If your unable to make the Realtek drivers work, then try some of the dozens posted on driverguide.com
You can find all of them in less than 30 seconds (unless you have slow speed dialup) by using our advanced driver search feature.
http://members.driverguide.com/index...on=search&sm=e
under manufacturers select ---> Realtek
under devices select ---> Net Adapters
under operating systems select ---> Windows XP
under model type in ----> rtl8139
click the SEARCH button at bottom and wait about 10 seconds or until page displays.