Win 7 should go to sleep like that mate. Not sure what caused it.
As long as the CPU is engaged in some form of calculation and not at all idle, it shouldn't sleep on you.

Mine has all the default settings, and will not go to sleep unless the computer is totally idle for 10 minutes.
screen saver ( I have that fire desktop one) keeps it form going to sleep for me even. so that's a good question.
I'll dig around and see what i can come up with.

g-man