Strip it down to the minimum. Disconnect any cdrom \ dvd drives. Take out all pci cards, but leave the graphics card if it has one. Just leave the bootable hard drive, 1 stick of memory, cpu with hsf and the graphics device. Give that a whirl.
If that will not boot, then check the psu power connector to the board and also the power lead from the case to the motherboard ( from the on \ off switch.
if that will not boot, take the motherboard out and place on a sheet of cardboard, see if it will go to post,it may be shorting on the case.