From what I have read, I believe it is a Dell... and from what I know, Dell's have there own plug on the motherboard for the power supply. Just another way they can screw a person over.
Yeah, it's somewhere on the motherboard. But I'm guessing it depends on what model of a Dell you have. I think the Dell computers [I.E.: Inspiron or XPS] is running a PSU with it's own Power Switch.