Could be a wide range of things. I am assuming you are using vista since this is a setting in vista that in the case of a crash it will auto reboot. My system does this but only when running a certain program. Might look into what your cpu usage is in task manager prior to it rebooting if the cpu usage is hanging at 100% then find the program causing this issue.
Do a complete virus scan as I had read a report that a certain virus will also cause this.
When it reboots do you get any reports of missing drivers as this will also cause this.