Download the SIS 900 version 1.19 driver for XP at
http://www.sis.com/download/
Unzip the drivers into a local directory like C:\SIS\
Reboot into Safe mode with Networking (press F8 at startup).
Uninstall the current SIS/LAN Card drivers.
Then in Control Panel choose to Add new Hardware device.
If you dont see SiS card in the list, chose an Intel Networking item from the list and browse to the XP .info file in the C:\SIS\ folder on your PC.
That should trigger the install list to show the SiS Networking adapter.
Install it then reboot the PC (into regular mode)
Fingers crossed it will work.