To narrow down.

Bad Powersupply

Bad Motherboard.

If the the CPU was out it would power on and not go to boot
If the ram was out it would go to post and error for bad ram
If harddrive was out it would post and say no harddrive
If card was out it would load up but no video.

So need to test the powersupply and make sure it's getting power at all points. I've seen them light up but not put enough power to drive the board. You could probably take it out and go to a local computer repair place and they could test it real quick just to be sure. I have a plug in meter to check them but I live too far away ahaha.