Computer power ON/OFF behavior is not as ...

My personal computer has recently developed a problem with its power on/off behavior. When I turn on the external power source (i.e. plug the power cable into wall outlet and turn the switch on) some components inside the computer turn on which is unusual. The power indicator led on board, the power indicator led on chasis, the PSU fan, the heat sink fan and the harddrive all come to life without having me to puch the power button on chasis. However, the system does not POST and no display appears on screen until i push the power switch on chasis.

Similarly, the computer does not shutdown as expected. When I shut it down via windows shutdown command, it ''tries'' to shutdown but at the moment the computer ''supposedly'' shuts down, the leds, fans and hard drive is still operating (it normally makes very faint sound when its spinning which i can still hear).

The motherboard is ASUS A8V-VM SE motherboard and the problem is either mobo or power supply related. It is probably not a faulty-power-switch-on-chasis; i tried disconnecting it from mobo and then turned on external power and again it turned on in the above mentioned state.

Any ideas about how to set it back to normal -- ie. the pc should not turn on until i push the power button on the chasis?