Drivers can be downloaded from
SIS' Download Center, or more specifically, the 'IGP Graphics Drivers, SiS650 & SiS740 series, Windows XP'
web page.
If the UniVGA3 Graphic Driver you mentioned downloading above listed SiS650 under Product Supported, then that should be the correct one. The zip file will contain an install program and/or instructions on how to update the drivers.
