A minor problem PC won't shut down its power in AHCI mode

[Castout]

When I reinstalled Vista, I switched to AHCI mode aware that Vista fully supports it.

However now when shutting down the PC, even though Vista has shutdown and the monitor turned itself off, the PC refuses to shut its power off.

I'm sure there's nothing wrong with my hardware, since switching back to IDE mode would make the problem gone.

I remember somebody here suggested that I might forget to connect my north bridge and my southbridge but I don't see where I must connect the two. I thought they are already connected since I can read and write to my HDD just fine. Meaning the CPU and memory in the north bridge could interact and function with the SATA HDD and my DVDs which are using the south bridge.

Does anyone have any idea what could be the source of my PC refusing to shut down its power? The atx works just fine because if I switched back to IDE mode instead of AHCI the problem would be gone.