PC keeps restarting by itself?

I manage to learn how to use a computer, but obviously I have no idea about its technical stuff except perhaps for very basic things. Anyway, the last few days my PC keeps restarting by itself. It is very puzzling to me, not to mention annoying and highly frustrating. It seems to happen when I am surfing the web, and it even happened in the middle of using the search function in myLot. I was also reading a discussion in myLot when it happened, without clicking anything or touching anything on my computer, all of a sudden the screen went black and the computer is restarting… What could be the reasons? Is it infected by some kind of viruses?