Inspiron 530S turns itself on overnight

My desktop Inspiron 530S is driving me nuts.  I shutdown every night and recently I have found the computer on and awaiting a password when I come to turn it on in the morning.  The monitor remains turned off. 

In the evenings I go to Start and "Shut Down" and wait until the blue light extinguishes itself.  I then turn off the monitor (rather that allow it to so to standby with a yellow light).  I do this as I am trying to turn off all the vampire electronics in my house when not in use.  Frequently, although not every time, when I return to the room we use for computers in the morning I see that the blue power light is on for the PC but the monitor remains off.  When I turn the monitor on, I see the screen asking for my Windows Vista password.

Any ideas as to what cn turn on the power switch, all by itself?

My wife has her own 530S and this does not happen to her and she does not turn on my machine.