This problem can be due to overheating of the CPU. When a CPU overheat is detected, this system might automatically shutdown.
Check that the CPU fan is installed correctly. Try unplugging the CPU fan and then plugging it in again.
Hope this solves the problem
I have a Siemens p.c with xp prof sp2 now a days when iam working its automatically switched off. I re-installed O.S, change RAM, Change Power supply. but still the problem not solved. please help me...