Screen powering on and then off

I got a mac pro 3.1 with no graphics card and no ssd(or hdd) i installed a nvidia gt 120 on it and i installed windows, it worked fine, but i wanted to try a mac os sistem so i installed OS X lion, in the installation the sreec kept powering on and off. Then i tryed installing OS X El Capitan, but unfortunately i still have the same problems