Xp is setup by default to reboot when a fatal crash happens
this could be due to all manner of reasons from hardware to dodgy drivers to dodgy applications.
if you can get the computer to run in 'safe mode'
press f8 when windows starts to get to safe mode
then got to control panel,system,advanced,startup&recovery,settings button.
then deselect automatically restart tickbox
xp will then give you a bsod which should help to identify whats going wrong
david
March 2006