|Sounds like a hardware issue or software issue.|
For hardware you could try removing the memory (RAM) and reinstalling it. It could also be a bad chip.
For software, there could be incompatible drivers installed. For starters, are you running Service Pack 3?
I've had a lot of experience with XP, and I do know that Avast 6, for example, will crash XP and reboot it continuously if you are not running at least Service Pack 2.
iMac G5 1.8GHz "Hero" PowerPC
2 x 1GB DDR PC3200 SDRAM
160GB WDC WD1600JD-40GBB2
Mac OS X Leopard Version 10.5.8