Why does my computer shut itself off without a request?

Hi friends please help me.My computer is shutting itself off.it just shuts off as if i were to press and hold down the button on the tower. it only shuts off at certain times. it will stay on for days if i only go to certain websites, but if i try to go play a game or something that requires more energy or time, i don't know, it shuts off. i ran virus scan after virus scan and it still does it. I don't know what to do. need help.

What kind of computer? and what Operating system? (Mac, Windows,
Linux, Be) There are lots of reasons for this, and not all of them
bad.

Does it shut off while you are using it, or after it has been sitting
untouched for a while?