as you have an XP CPU you will have a Via chipset i would think go to their wedsite and download the hyperion chipset id drivers onse that is done do the following
uninstall everything to do with your G.card
then install the chipset id drivers when you can select normal mode for agp driver NOT turbo mode
let it restart when prompted then install G.card drivers but get the latest drivers.
if its not that i can only think of 2 other things
1you need to flash your BIOS to get enhanced AGP support
2 the power suypply itself is not powerful enough or you may need to increase the power to your CPU a little