Question : Problem: Why does computer shut off before post

Compouter went through a spell where it shut itself off every forty minutes or so. Windows XP SP2 seemed to go through an orderly shut down, but there were no relavent system messages as to what happened.  Now when I turn it on, the Dell logo screen is up for a split second, - then the compuuter is off before a complete POST, I have replaced the power supply and motherboard. Now looking for ideas.

Answer : Problem: Why does computer shut off before post

Dell technician came in. Replaced power switch, pwr supply, and mobo. Ths was a bit like using a bulldozer instead of a shovel. I can't be sure what the problem was - but it's gone now.

Thank you all very much though.