PC shut down by it self.

My brother, bought a PC pentium 4 1.7 GHz, with 256 memory. 40 gig hardisk. Last 2 years. Lately the computer always shut down by itself and unable to rebot back.I checked the system, and found out that, when the shut down by itself the pc power is on. I tried to rebot back by restart button or start button it unable to on. I also unable to off the PC, by pressing the start button for more than 5 mins. The only way is to off the main switch and press the start button, than the will PC on.

Pls help

Regards
Sani