Unexpected Shutdown

Hi

I keep experiencing problems when i turn on my pc, sometimes it loads fine and then turns off automatically on its own. My system does not have any spyware viruses or other malicious form of infection. The PC no longer turns on as it says that a system file used to load the system is corrupt. I attempted to re-install windows XP and while i was in the middle of set-up the PC shut-down again. I have no idea of what the problem is but i can no longer turn on my pc properly because there is no operating system installed and when i attempt installation it turns off

any answers would be greatly appreciated.

as you may realise im using a different pc to ask the question

Many Thanks

Jamie