First make sure that the chipset on the card is a RTL8139(a,b,c) not a RTL8129. (the chipset information is located on the physical card, stamped on a small 1 x 1 inch brown chip)
If the chipset is the RTL8129 then go to the RealTek website to get the driver for Microsoft Windows XP:www.realtek.com.tw
If the chipset is one of the the RTL8139 versions:
1.Go to Device Manager and remove the driver (AT-2500TX that would be listed under Network Adapter (NIC)s); DO NOT restart computer
2.Run atsetup.exe which can be downloaded from our software website
3.When you get to the ! Warning "The Software you are installing has not passed Windows Logo testing to verify its compatibility with Windows XP" click "Continue Anyway" and if the warning appears more than once just click on "Continue Anyway". It will install the drivers
4.Restart the computer and everything should now operate correctly.