Dell Optiplex 780 Won't Boot Past Loading Windows Screen

Hey guys,

        I have a Dell Optiplex 780 tower and I was getting these beeps a couple weeks ago. Computer wouldn't turn on at all. Turned out the beeps were from memory modules not being seated properly. All fixed. But now, Windows will not start up at all. BIOS loads, then the "Loading Windows" screen with the spinning 4 colors and Windows symbols, then it flashes blank black screen and nothing. I am running Windows 7 (computer came with Windows 7 preinstalled and included disc for it). I even tried a brand new hard drive and installed Windows 7 to it but same exact issue. I am getting no more beeps or flashing numbers during start up. Screen just goes blank after initial load. Any idea what it could be? Thanks in advance!