Wait... your PC is shutting down because of the heat?  How hot does it get before it shuts down?

I did some test runs on my PC to see how hot I could get my 5850 before it would get "SICK" in CGMiner, and it seems like I can push it to about 95ºC before it turns off.... however, my PC never shuts down.

Oh... lol... turn crossfire off!  Hope that helps.  You don't want crossfire on if you are mining!