I have a problem similar to the questions posted here and here. This is my config:
Asus M3N78-EM, with AMD Phenom X3 720 2800 Black Edition, 4GB Transcend DDR2 RAM, Nvidia 9400GT. HD is a 160 GB IDE, and a LG IDE DVD-ROM. The power button is a bit off, I have removed the cover of the switch, and the only way it turns on is just giving the "stick" under the cover a gentle press. It turns on sometimes, and at other times, I have to cut-off the power from the PSU, and try again.

I will describe my problem in as detail as possible, please bear with me: The problem has started in the last week, a few months after I changed the to the powerswitch arrangement as described above. The PC makes a hissing noise, and I wasn't able to pin-point the noise source, because of the various other fans. At first, removing the HD, rebooting w/o the HD, turning it off, reconnecting and booting made the problem go away. But of late, it doesn't happen. As suggested in the other questions, I tried reducing the load by disconnecting both the IDE drives, and the problem (noise + turn-off) still occurs. I also connected another 80G IDE HD,today morning, adn it still made that noise, and turned off.

I also opened up the PSU, but I couldn't see any fault in that, I tried rotating the fan by blowing into the blades, and with my fingers, but the hissing noise didn't come from there. Or maybe the speed wasn't enough to evoke that noise.

A few weeks ago: I had cleaned the Cabinet and had repasted the processor and its fan using some thermal paste. Could that be at fault? I also used a vacuum to blow the dust out of the PSU, could the power have been too much, to maybe offset the fan or something? A label on the PSU says it uses a ball-bearing fan.

That only leaves me with the Processor fan and the processor itself. I didn't try removing the processor fan and processor from the motherboard, and then turning the PC on, fearing damage. Will doing so cause any damage? What can I do to localize and pin-point the problem?

Also, after a few tries, the Computer starts up. Sometimes it turns of within 2 seconds, sometimes after the POST. Once it turned off at the grub. Another time it booted completely and then turned off. The only way to ensure that the PC wont turn off, is if the hissing noise stops.

EDIT: I suspect it to be the Processor/Processor fan, owing to the source of noise. All the config, except for the Cabinet, is just over a year old.

EDIT2: I also just remembered, that I had set the "On-power resume" to turn on, i.e. If I supply he PC with power, it will turn itself on, w/o me needing to press the switch. I had done that to workaround the faulty power-switch, as noted above.

EDIT3: I calculated the power my system needs, from the antec site, and I just arrived at 292W