Windows 8 Problems

I installed Windows 8 about 1 or 2 weeks ago, but the last time I used it, it was unstable. All there was is a black screen. And when I tried to start task manager, it just looked like it got sucked in by a black hole. I pressed Ctrl-Alt-Delete and pressed the power button, but it said "Cannot start Security Utilities.

When I Signed out and tried to sign back in, the screen keeps on flashing when I press a button. The only way to shut down is to hold down the power button.

Before it was fine, but right after I installed Avast! Antivirus and restarted, that's just what happened.