PC Starts then switches off

Have a PC AMD2000/Soltec MB/500M mem/ ATI 7000/80Gdrive/winXP

It starts then swiches its self off before windows starts with nothing showing on the screen to indicate a problem.
So far I have

1.Checked seating of all cards/ Chips etc.
2.checked the mem with memcheck
3.Tried a different PSU.
4.Tried to loadf XP onto a new Hard drive (goes through initialisation then a click and switch off :(
5. tried to install windows Me which will some times start in safe mode.
6. Tried to set the Bios at default settings.

Only bits left are MB/ CPU and Vid. Anyone any inspiration.

Kev

Check the shim is on straight (if there is 1, is usually the problem with mine.

Ben

Check to see if heatsink is seated on the CPU correctly, this will cause the symptoms you just mentioned.

Memory and GFX missing, not seated right, not working, will make the mobo give out a POST beep code, and will not power down your system.

Hmm Shims -what shims?
Kev

if there is one he said ;)

MANY THANKS PROBLEM SOLVED -
the cpu heatsink was fitted wrong way round (not by me) so that the thermal pad was only touching around half the cpu. I understand its worked for a year like that. Dont know how that caused the pc to switch off on bootup tough.
Kev