Might be short-circuiting,
It might be the motherboard short-circuiting on the metel computer housing, try looking where the motherboard sits in the case, make sure that no part other then the screw holes are touching the metel case.
First take the motherbaord, Power Supply Uint, Hard Disk Drive out, then rebuild it on a table then try turning it on short the power on by touching the power button prongs on the board (both the + and -) with a flat-head screw driver, if it is short-circuiting in the case, it should not short-circuit on the table.
Let me know how it goes, this is an intresting thread for me.
BTW, please put more then "help" in the thread title next time I.E "Help with overheating".
P.S Default the BIOS Settings as well.