Keyboard/mouse powers on computer after shutdown with Vista

Hello!
I'm having a weird issue with Vista and my Asus V3-P5V900 (this a
barebone machine).
Everytime I shutdown the computer, I can power it on with just a key
press on the keyboard or clicking the mouse.
Yes, I'm sure I've turned the system off and no I didn't suspend it.
I've tested this on two machines with this motherboard but with
different CPUs and hard disks. Both keyboard and mouse are usb. Also
with Vista Home and Premium.
I'm pointing to Vista here because I booted off an OpenBSD install CD,
shutdown the machine off and it didn't came back on by pressing the
keyboard.
I'm thinking Vista "enables" something on the motherboard when
shutting down, so it can be powered up by USB?
I couldn't find anything on the BIOS apart from the "power on from
keyboard ps/2" but that is disabled.
I've also disabled "allow this device to bring the computer out of
standby" from both keyboard and mouse in device manager.
Any comments are welcome. Thanks.