Actually that sample application is far more complicated than you need since it sends the output through an LM131 voltage to frequency converter.. If you were to send the output voltage from the LM35 through a variable resistor to a transister that flips a relay switch you could calibrate the variable resistor so the circuit triggers the relay at whatever temperature you choose.. You could determine that temperature by taking voltage/resistance measurements at the triggering point, but its easy enough to calibrate it to work by trail/error alone.. All you need is an LM35, potentiometer, basic transistor, and a relay that is rated for the current your fan draws.. Add a couple cheap components and a 555 timer if you want it to run for a specific time then shut off, or any other tweaks/addons..
Circuits like this are all over the web since alot of ppl need to control appliances for alot of things..