My computer shuts down only seconds after i turn it on. What could be the cause?

While i was playing diablo II the sounds stopped and the speakers made a crackling noise while the screen froze and the computer shut off instantly. Now every time i try to power the tower on it shuts off only a couple seconds after i push the on. The screen doesnt pick up any signal the few seconds its on. Ive already changed the power supply but the exact problem remained. I also removed the sound card and changed the gfx card, still no signal plus the powerdown