PSU issues rarely beep. The only real way to eliminate a PSU problem is to try a different one. For all we know a 12v rail could have died so there isn't enough power to give video to the screen. I have seen this be a PSU many times in the field. Also remember, age means nothing when it comes to if a part works or not. Some don't work strait from the factory, others die in a few minutes, others weeks, others years. It's luck.
I think AR is right on the ball with suspects. The only thing that we have crossed off the list of possibilities is the HDD. Look at Motherboard, RAM, and PSU as the most likely causes.
The motherboard may have physical signs of failure. Look at all the capacitors on it (they look like real small soup cans), if they are bulging at the top, blown open or leaking your motherboard is bad.
