PC wont turn on, new power supply, motherboard & more

It is not the switch or the cd rom, substituted both. Still remains a megamystery, everything was replaced, power supply, motherboard, switch,
disconnected memory sticks temmporarily, no effect.
Only clue I have is when I plug it in, the fan starts up, (not supposed to until I hit the on button) then shuts down again.
GT4016 desktop

 

It sounds like a bad connection with the wireing.
first you say the motherboad was replaced if it is shorting on the case because of a bad standoff this will cause that prolem also check all the connections it is not possible to connect to the wrong plug on the motherboaed because the pins are set to fit only the correct way but if they are bot pushed fully home this could also be the problem the white socket on the motherboar must be fully pressed in.

 

You could also try pulling the CMOS battery to reset the BIOS. Sometimes a "hard reset" works...