Non-bootable computer

I'm trying to help out a friend of the family's computer. The problem is, it just won't boot at all. I first thought it may be a power supply issue so I purchased a new power supply and still a blank screen. I also swapped out the graphic card into another computer and ran into no issues there. I re-seated the memory in its slot and also re-seated all the internal wires and still nothing. What I find peculiar is the hard drive LED will come on but the power LED will not. Also, when I turn on the switch for the PSU the the case and processor fans will start spinning but I cannot turn off the PC with the power button on the front. Thanks in advance for any attention given to this thread.