If there is no problems found with any hardware like a supply with a weakening cap or memory faults starting to be seen. A possible weak battery after a year's use can come up. A need for a bios update? Besides the hardware side of things the rest is software related like a driver fails to load correctly kicking in XP's crash control there.
The next time you boot the same driver loads normally. One thing to look over is the list of items seen in the msconfig utility's startup group if there's a lot of programs installed. You can often debug a system by simply disabling too much from loading at the same time as Windows does.