My Toshiba laptop keeps freezing up no matter what. Even if I click "Ctrl + Alt + Delete" nothing happens and I have to press the power button to shut it off and turn back on. Also, a few times the screen will just turn black (but not off- its like a lighter black color) and ill click on the mouse or tap the power button and nothing happens. It's not like its going into hibernation where you just tap a button. And again i would have to shut the computer off and back on through the power button. I scanned for viruses and theres nothing. What is it? The computer is 4 yrs old. It's not the battery because I have good power life and/or its usually plugged in. It's not overheating either.