check the internal temp. Use a program like speedfan or something similar to track the speed. my computer runs idle at about 35-40 degrees C. Are you doing things that require a lot of processor power when your pc crashes? On the note that heat could be the problem, check the BIOS to see if there's an automatic shutdown temp.
Also, check your RAM modules to see if one may be defective. If you have 2, try running one-at-a-time and alternate if it still crashes. borrow a ram module from a friend if you only have one stick.
You could also try putting the ram in different banks. Some motherboards work better if you put the ram in banks 1 and 3 instead of 1 and 2, for example.
You could see if the power supply if a problem if none of the above work. Just swap it out.
If none of that works, you may have a corupt Windows installation or some 3rd arty app is mudding things up. Start with uninstalling programs that you've installed when you first started noticing instability. Worst comes to worst, you'll need to reinstall windows, but that's always fun