In the bios itself you would set power management to user defined as a rule. That allows you to enable/disable sleep mode and other settings in the control panel. But that setting alone wouldn't cause a freeze up. If the battery was weak then the bios programming would be effected.
You would probably see problems booting up there however. The one thing now would be a possible IRQ/resource conflict due to XP's resource sharing. The last is one you won't like being a failing chipset if not one of the devices you have connected. Have you tried running the system with no external devices hooked up?