Also exactly what happens when you turn it on. I pressume you get a green light for power? Anything momentarily displyed on screen? Blank? Black?Blue?
Im guessing completely blank screen.
Then what? The green light stays on? Does the HDD indicator flash like its trying to access it? Is the fan still running?How does it turn off? By itself or you press and hold the power on/off button for few seconds?
Any info would help.
Its unlikely to be PSU. (a) its powering on and staying on, (b) you changed the psu. You could check a few voltages on the main board to make sure you getting +/-5V or +/-12V, but I think its safe to say PSU is fine.
Are you 100% sure its not something stupid like a dead monitor/display? ie it is booting you just cant see it?
Im assuming its a desktop.
You really only have monitor + CPU
In the CPU you have some periphial boards like sound card etc. The PSU, RAM, HDD and the mother board.
The periphials wont be an issue. So if its not PSU or monitor, it doesnt leave much.
Your confident about the RAM, im not so sure if its not even getting to the point of counting the RAM on the screen. You probably have some onboard RAM too, so could try taking all 4 RAM out.
You really just need a working computer to rule out everything 1 by 1 IMO.
If you can confirm RAM and HDD in another PC, then all that is left is an actual fault on the mother board which can happen and you arent going to repair it at component level. Try a good clean, reseat everything and then bite the bullet and replace the board.
Its not hard to test HDD and RAM, all the IDE connections are same.