I'm gonna go out on a limb and guess that its time for a new video card.
This sounds extremely similar to what happened to my computer in mid-December. I had 2 video cards (for SLI) and one died and was preventing the computer from even booting up. I did not know that was the problem and and thought my hard drive was corrupted or infected (it wasn't but I re-formatted just to be safe).
I removed both video cards and then put them back one at a time and booted up. The first one emitted a sound like a smoke alarm (long, shrill high-pitched noise) and would not boot up. Took that one out. Put the other one in and the computer would boot up. Most of my data was still on an external backup drive (which I had just bought a few months before, thank jeebus).