Computer shutting down after 5 secs...cannot identify problem.

Hi, great forum..wonder if anyone has any ideas on this issue i am having.

My friends packard bell UTOW D10 wont boot up. Motherboard is MCP73VT-PM Processor is intel core 2 quad core 2.4ghz with 250w fsp power supply.

The sytstem turns on and the cpu fan is working then all of a suddent the computer shuts off after about 5 secs, no beeps and nothing to the monitor. I initially thought this to be processor issue as no beeps being issued. I have systematically replaced cpu,motherboard and power supply with identical parts and the computer is still doing the same, cant understand what it could be. luckily hdd is ok and have recovered data. The ram also works fine in another pc so is not that either. Any ideas??????