i just saw this thread, and as Null said, put HD into another PC...anyway, you said you tried another power supply and it didn't work? might not be the problem with that, might even be your Motherboard, although if the light on it is lit up, there is no reason why it shouldn't work.
as for Null: if it was the RAM, the pc would still turn on and at the bios boot screen would beep a few times saying that there isn't nay RAM or not enough or whatever...but the RAM would have no reason for making the computer not turn on at all.
and sorry i'm late
...was playing Rose Online...and i didn't see this thread when i did check GT...