My computer won't start.

I have an HP Compaq Presario SR5060AN Desktop Computer, and last week, after restarting my computer, nothing will load. When I press the power button, the fans start to whir, then go silent. The fans are still running, but nothing at all loads. The screen doesn't turn on, I've tried multiple screens, so that's not the problem. Put 2 working sticks of RAM from a similar computer (SR5160AN, to no avail, same thing happens. Reset the BIOS settings from the Motherboard, still nothing. I'm not sure as to put it down to hardware (Motherboard, CPU, Vid Card, Power Supply?) or software. Since nothing at all is loading, I have a feeling it may be hardware, but I'm unsure as to just what it may be. What I'd like to know is if anyone could point me in the right direction to find out the source of the problem, so I know which part would need replacing. I'm not keen on sending it away to a technician, just so they can charge me an arm and a leg to tell me what the problem is... without even adding the cost of fixing it.