Unexpected Shut down of PC

I had PC Celeron 667 with Intel 815 original Mother Board. 128 MB RAM. 52x LG CD ROM Drive. FDD 1.44″. Kodak 8x CD Writer. Intel PC Web Camera. Sharp AJ 1600 Printer.

My Pc shut downs unexpectedly. I had checked all plug connections. It is OK. Ihad formated the hard disk with doubt about software. But still after that it is shutting down unexpectedly. OS is WinXP.

Is it due to over heat? or SMPS problem? or Mother Board? or Power Management?

I broke my head and not able to understand.

Anybody can help me? I am not an expert or qualified technician. I am a home user with practical knowledge of computer.

Thanks for help in advance.