Computer randomly shuts itself down.

I've been having this problem for a while now and I've been leaning towards the fact that it's been overheating, but I left it on for a while so it could finish its virus scan last night and it shut itself down again. I guess the explanation could be overheating, but it was relatively cool last night compared to the afternoon and the only thing running was AVG, I don't really see how it could've overheated. The scan turned up no results, but it was incomplete.

Could someone diagnose the problem for me or something?