My computer crashes after only a few minutes after going ingame.

A few minutes after i get into a LoL game my computer crashes and I'm forced to turn it off manually. When it crashes i hear a monotone sound and my screen goes black. I can have my League client up for ateast 10 hours without problem.

I haven't done the finest job in cleaning my computer, but I've tried playing other games such as Civ, Skyrim and Deus ex, as well as watched worlds without any problems. Strangest of all, I've re-installed the game and it doesn't help.

This problem didn't start with the new patch, it started happening today and since then it has happened every game, every 2-3 minutes.