Computer works fine, but won't stay turned off completely?

Hello everyone, thank you in advance for any help!

I home built my brother a computer almost 1 1/2 years ago and have replaced the motherboard (Asus P5Q Deluxe) three times (third one works while turned on) and the power supply once (Corsair 520HX to the 620HX), my YouTube video shows the problem with shutting off. The first two motherboards the whole computer would not post, the third motherboard works flawlessly while using the computer, but ALL of the fans either stayed on never turning off or surged on-off-on-off over and over until I pushed the switch on the back of the PSU. Why am I having the same issue with three motherboards and two different PSU models? I will list the specs of the computer below, note that I have tried the EVGA 9600GT, G.Skill DDR2 2x1GB, Western Digital HDD in my computer which works flawlessly! The only thing I have not tried in my computer is his P5Q Deluxe mobo, Corsair 520HX and the 620HX PSU. I'm hoping it's just a BIOS setting, I really am! I have also assembled the motherboard outside the PC case with the same problem!

http://www.youtube.com/watch?v=kUfIJS_hU4E

Never had any overclocking!
Asus P5Q Deluxe
Q6600 stock clocks
G.Skill DDR2 2GBHZ 4-4-4-12 2.0V
EVGA 9600GT
Corsair 620HZ PSU
Western Digital HDD

I watched your video, I want to be clear on what I saw at the end. You flipped the PSU power button OFF and the computer still seemed to have lights and fans spinning. Is this correct?

If it is the powersupply is faulty.When you flip that switch no power should be supplied to the PSU.

It might be a pain in the butt, but I would physically unplug the power cord from the PSU. It shouldn't turn on at all after that.

I would hazard a guess that your power button on the case in front must have a short and can randomly make a connection or your low voltage pins down by the videocard may have bent connectors possibly touching each other.

I always shut down and unplug the cord during severe weather so there is absolutely no chance of a lightning power surge getting into my rig. You may have to just unplug the cord everytime you want to turn it off!!