ok the best thing to do at this point is to get the PC down to the minmum required hardware to run to isolate the problem. Use 1 stick of
ram no graphics card(use the onboard one) unplug all hardrives cd/dvd roms floppy drives and any other cards you have in the PC and turn it on it won't boot of course but will it stay on then?
Some times it is best to remove the mainboard and power suppy from the case and just fit 1 stick of
ram in turn it on and see if it will stay on. Many people make the mistake of puting a mainboard mount in the wrong place causing a short.