Even when i've fully shut down my pc at night, (but not unplugged from the wall), a green light comes on at the back of my tower near the main cable. It doesn't always come on right away, but can sometimes be hours afterwards...it's even woken me in the early hours with it's unearthly glow coming from the back of the pc! What can it be when everything's shut down?!
Is this green LED internal within the PC or mounted on the case like it is supposed to indicate something? If so is there any symbol nearby. Anything to do with network activity, do you have it connected by cable etc... just guessing. Uninteruptible power supply... have you got one of those? Is it a full shut down you do or sleep or hibernate instead.