There are two Ethernet cards, Unknown Realtek hardware Atheros AR8121... There is nothing wrong with the Atheros... But there is a big problem in Unknown Realtek hardware.

I don't know whether this exists. Because ID(Unknown Realtek hardware - VEN_10ECDEV_0139) have no existence in all of the Ethernet cards. Maybe your card is just displaying the wrong vender or hardware ID. If you know anything about 'Unknown Realtek hardware', please let me know. Those drivers could be changed or updated at any time.

Read this page for your guidance : http://www.computing.net/answers/windows-xp/windows-xp-pro-and-lan-card-wont-install/5413.html