I'd try another motherboard. The problem seems unrelated to the new HD.
As of exactly a week ago, I bought a brand new HDD and reinstalled everything from zero. I used the back-up disks to go back to factory settings. This morning my mother turned on the computer and what she said is that the computer started normally up to the log in screen, only that there were no users, but a message saying that there is something that needs to be installed, and that the PC will restart automatically. When I woke up, the PC was turned off. When I tried to turn it on, the lights would go on as usual. All fans work properly, only that the monitor says "No signal", and the mouse and keyboard have no power. I tried taking out the RAM, and yes, there is the beep from the motherboard. I tried changing the monitor, same result.
I have an HP Pavilion Elite HPE-410f, if that helps.