If what you say is true then all I can think of is the motherboard. I am the guy with that computer, until now I tried only with the PSU and the hard disk.
But I gave up even to think about.
In the meantime I bought another comp, way much better.
It would be good to change the motherboard too, to try the original pieces with another motherboard. Unfotunately I can't do that because I can't get any other piece.