rebooting randomly

Hi there! I'm getting kinda desperate here. My PC restarts randomly, no matter what I'm doing. It does it when it's on idle, when I'm googling something with only Opera open, when I'm playing a game, it even restarted 2 times already while I was writing this on the NotePad, it just doesn't...

Heyo! So, my PC is like 2 years old, the matter is, some months or even a year ago she started to reboot, and that's all, just rebooting. I remember exactly this happened after some Windows Update, or likely, but I can't figure what update was, or why Windows could cause this, but: Over this...