You couls try an Analog Devices ADT14gp.Its a temp monitor with a programmable output.Feed the output into a 555 PWM circuit. Depending on your knowledge ,you should be able to do it. Most people start off destroying 555's.

But the best solution is the Zalman Flower Fan...I got really pissed off with the noise from my CPU fan,so I fitted the Zalman and binned the fan completely. Eutopia - No noise at all...