I hope it's okay to post this here. I found this free program very helpful so I wanted to pass on the info.
I just downloaded a free heat checker and it has an alarm you can set for when your temperature gets too high. I really appreciate its help, I can tell you, when I'm in the middle of playing Sims 3 and forget about the time.
Link: http://www.techpowerup.com/realtemp/

Ooh, that's very handy, thankyou. It's worth remembering that it only monitors your CPU though - so your graphics card could still be overheating and you wouldn't get a warning. So it's a good idea to also monitor your temperatures with something like HWMonitor if you have a mediocre graphics card.

You can use GPU-Z to monitor your GPU temperatures.