A thermistor is a component which has a roughly linear resistance change (or at least predictable) response to variations in temperature.
A fixed current flowing through the thermistor will thus be altered depending on the ambient temperature. What I suggest you do is find a simple low cost microcontroller with built in ADC (analog to digital converter) ports. A simple external circuit (probably described in the microcontroller's datasheet) will allow you to digitally make measurements of the current through programming instructions. The resolution will be dependant on the bit depth of the ADC, but typical values in simple microcontrollers may be 8, 10, or 12 bits.
Then, regardless of whether or not you have a datasheet on the thermistor, you can make a chart of the digital number associated with a wide variety of test temperatures and ultimately form an equation which models the temperature.
Something such as the pseudocode ...
unsigned char measurement = read_ADC;
temperature = convertToCelsius(measurement);
... can be used in the program to easily allow manipulation of the data.
The a simple while loop can constantly check for new temperatures and respond via if and then control statements depending on preprogrammed thresholds.
You can simply end up making a device which triggers a relay if the temperature is greater than x but less than y, for example.
The relay can control the fan to begin the thermostatic process.
Sincerely,
Jesse Randall
Electronic Engineer
**broken link removed**