My PC turns off time to time. I don¥t know why. It just turns off.
Is there any trend of what you're doing when the computer turns itself off?
How long is the computer typically running before it shuts itself off?
Will it turn on, no matter what you're doing? Does it only shut off when doing something fairly CPU-intensive? fairly graphic-intensive? Or will it shut down even after a period of time, at 0% or similar CPU usage?
Kor, you might be on to something.
I'm wondering if it's a hardware or software issue. Do you know how to check the temperature of your computer's various hardware? If you can take a look at that right after your computer shuts down, it may or may not give you a clue.
Maybe or maybe not a CPU overheating, but there are other components that can overheat as well: video card, power supply, motherboard, hard drives (probably not hard drives though in this case).
Just a thought, but take a look at your hardware's temperature and see if it could potentially be related. Your computer may automatically shut down to protect itself, or it could be shutting down due to a hardware failure at high temperatures.
Do you have a program to monitor your computer's temperature while it's running Windows? I like Speedfan for this purpose.
Not all computers work perfectly with Speedfan. You can see my desktop, it thinks I have some sensors that I don't and speedfan reports weird values on those sensors.

For the most part though, it's very useful.

Yeah, my hard drives can probably fry eggs after awhile.

(though for some reason, HD2 is nearly 10 degrees cooler...

)
Temperature may not have nothing at all to do with it, it's just a theory.