The computer isn't much better than last time. I'm running an Asus
A7V8X-X mobo. As soon as I flip the power switch on the ATX PS, the
whole thing turns on (fans start spinning, etc.), which is bad, because
usually the computer waits for me to press the power switch before
turning on. I've reseated the mobo, re-checking the metal stand-offs.
Those are fine. I've reseated the CPU, heatsink, video card, and
memory. Still, nothing at all happens - it doesn't post, and it never
sends a signal to the monitor. I know the CPU works, and the RAM and
mobo are both new. Any clue as to why this computer won't boot at all?
Why it doesn't even recognize the power switch and just goes full-on
immediately? Thanks in advance for the help.

I take it that you've tried the suggestion given in a reply to your previous
post of reversing the cable from the power button on the case to the
motherboard? As was pointed out, if this is the wrong way round the
motherboard thinks that you're holding the power button in continuously - so
when you turn the power on at the PSU it comes on immediately, and then,
because you've (apparently) been holding the button in for five seconds it
goes into the soft-off option, and turns off.

For reference, the other common problem from having those cables the wrong
way round is the reset cable being reversed - so the computer constantly
resets and so (obviously) won't POST. Sometimes a tricky one to diagnose -
unless you happen to think of it...