ok, i have finally got round to sorting out the screenshots for you. here is the simple circuit:
here is the simulation of the fan not being on because the temperature is low (30degrees in this case) :
and here is the simulation of the fan being on because the temperature is high (40degrees in this case)
And by changing the values of the resistor, and using a different sized thermistor (as this one only measures up to 40degrees) you can change the temperature at which the fan turns on.
hope this helps in some way or another