I just made it on to XP Pro this week, and am SW:GB running fine.
My system IS a via, a A7V266-E board, Athlon-c 1.4, 512mg ddr, Visiontek G3 Ti 200, onboard sound, latest drivers. Things have been stable, except one issue with IE 6.
Ideas: try a different/older driver for the Nvidia card. I've read some reports of people trying dif drivers to get SW:GB, AoC to work on XP.
I also don't trust any card by Creative after the SB Live! The SB series experience errors with all vendor chipsets, not just via. However, it is said that all outstanding issues are fixed in the Augidy series.
forums may have much of interest for you. Try there.
It is always worth checking your main memory. Download a free memory checking pgm here.