Vista - Screen keeps blacking out

I have some other problems associated with the screen blacking out after I've typed in my password on the welcome screen and entered it. It then brings me to my account, but if I don't start clicking on icons (I usually click on Mozilla Firefox) to get the computer going, the screen blacks out on me. Why does this happen?

It's not a hibernation problem, I don't think, because I've set up the power options so that the computer hibernates after 15 min of idleness, but I haven't even been on the computer for 15 min, and the computer shuts off. The screen goes black, but the light bulb and battery meter lights are still on green.

Another problem I have is that it takes me around 15-20 seconds (sometimes less) each to press the power button so that the computer may shut down/turn on. Also, if I don't pres the button long enough, the computer may not shut down or it doesn't fully turn on. The light bulb would turn green, but the screen remains black. This didn't use to happen. Neither did the black screen coming on now and again. Even as I type this, I'm afraid my computer will black out on me suddenly.