Helping people with computers... one answer at a time.
A black screen on your computer can mean several things. A black screen is most commonly a screen saver, but there are also much worse possibilities.
My computer switches to a black screen after about 5 to 15 minutes of use. Then I have to turn it off to get another 5 to 15 minutes of use. Maybe someone could just post what part of my computer is causing this black out.
This is another of those questions that I get very, very regularly. Unfortunately it's hard for me to offer concrete help. The problem is that there are so many possibilities, and there's no single "part of my computer" to point at. There are many things to look at, and look for.
It's probably obvious to you already, but I'll say it anyway, it could just be a screen saver. If you wiggle the mouse or hit a key on the keyboard and your screen comes back, that's it. You can adjust the screen saver behavior in the Display portion of Control Panel.
If you have a laptop, it's possible that the computer has gone into standby or hibernate. If you simply push the power button and it comes back up to where you were (possibly after a few seconds), that's probably it. Normally a computer should only go into standby if you're not actively using it, but if your battery is very low, it my force standby in order to avoid losing what you're doing.
Those are both normal, and expected behaviors. What's left is typically an indication of a problem. In my best guess "order of likelihood":
It could be your computer. The most common abnormal reason for a black screen is probably a computer crash. Given that it's consistently 5 to 15 minutes after boot up, I'd be looking at possible overheating, or perhaps a power supply that's going out. One simple way to determine if it's just the monitor going out or the computer crashing is to play some audio on your computer. Perhaps load up a longish mp3 play list, for example. If the monitor goes black, but the music continues to play, you know that the computer is running, and that the problem is either the video card, video drivers, the monitor or the cables in between.
It could be a virus, or your computer's other software or drivers. Unfortunately bad drivers for any hardware on your system can cause the machine to crash or go to a black screen without warning. I've also heard of this as a symptom of a virus (though typically a poorly written one). It's more common that you'll actually get a message, or the dreaded "blue screen of death", but nonetheless, it's still possible that bad software, particularly hardware drivers, could cause it. As always, make sure your computer is up to date on Windows, and of course anti-spyware and anti-virus software as well.
It could be your video drivers. Even though I've already mentioned drivers in general, the video drivers are worth a closer look. The software used to control the video card, which can often be quite complex, can also fail, have a bug, or crash in such a way that the screen goes black. Most commonly this'll crash the computer with some kind of visible message as above, but it's also possible that the video card drivers or software could fail, and the computer keep running. Certainly making sure you have the latest drivers always makes sense.
It could be the video card in your PC. It could, for example, be overheating, and as a result cut out after 5 to 15 minutes. Turn it off, it cools down, and you get another 5 to 15 minutes of use.
It could be your monitor or display. Monitors and LCD displays can break, and one way they can break is to go completely black. And yes, sometimes they'll even come back on for a while, though it's typically not as consistent as you've outlined in this particular question. The best way to test this, of course, is to try a different monitor for a while.
It could be your CRT's circuitry. Many CRT monitors often have their own standby circuit - if they notice that the signal has gone, they also power-down. If that standby logic fails, they could possibly go into their own standby state improperly. Usually there's a standby switch that, if you push it, causes the monitor to spring back to life.
It could be cable between your monitor and computer. Again, this typically isn't as consistent with respect to time as you describe, but a failing monitor cable can definitely cause an intermittent display, or cause it to go to black.
So as you can see, it's not a simple process to diagnose. Some steps, like trying another monitor or cable, are pretty easy for you to take yourself. But if you're not comfortable diagnosing system crashes or hardware problems like potential overheating, it might be time to take the computer in to a local technician for a hands-on diagnosis.