Question : Power down; fans & USB stay on

My power down/power up procedure in Vista 32 goes as so:

1. Power down in Vista.   Computer appears to power down normally (monitor loses signal, drives stop spinning, etc) except fans and USB devices stay on.
2.  Holding in the power button doesn't shut the system down at this point.   I must turn off the switch on the power supply to shut down everything.
POWER UP
3.  Turn on the power supply switch.   Push the power button on the tower.   Fans & USB come on; computer does not.
4.  Hold in the power button.  Everything powers down.   Hit the power button again, and everything powers up like it should, and I go into Vista with no problems.

I'm using a P5N32-E SLI mb.  

Any ideas?

Answer : Power down; fans & USB stay on

I would try S3 only.
If that doesn't work, try Disabling the main Power Management setting.

The Sx meanings:
S0 - system ON
S1 - light Hibernate, fans keep running, ram still ON
S3 - heavy Hibernate, everything is OFF, but ram.
S4 - fully OFF or Hibernate.

Make sure the Vista Power Management is set the same as the BIOS. You can also disable one or the other, and use just the hardware (bios) or software (Vista), if you want to use Sleep/Hibernate on your system.