You need to specify the voltage and current rating of the fan motor before any meaningful help can be given. It would also be nice to know what kind of motor the fan has - brushless DC, series wound, shunt wound, etc. You may be blowing fuses because you are trying to apply too much voltage across the motor windings, and/or because of the type of motor that it is. If you persist, you may smoke your motor windings.
In general, a simple circuit that just varies the voltage to control DC motor speed is not the best way to go. The motor needs some current to generate torque to overcome the inertia of the fan blades and any bearing friction to get started. Until the voltage and current is high enough to get the rotor moving, you will just heat up the windings in the motor; how high the current will be and how much heating occurs depends on what type of motor you have. Once the rotor is moving, it generates a counter EMF that reduces the running current, which in turn reduces the heating in the windings, etc.
A good motor control circuit, that really isn't too complicated with inexpensive IC's, is a pulse width modulated motor control. This works by applying a constant operating voltage to the motor, but varying the speed by controlling how long the current to the motor is turned on. It switches on and off very rapidly so you don't notice the cycling, but the motor speed is proportional to the average time it is 'ON'. The on time is determined by the width of the pulse applied - hence the name pulse width modualation (modulation is a variation in a signal).
An example of such a circuit can be found at:
http://www.solorb.com/elect/solarcirc/pwm1/
Note that you could use your 25VAC motor to power the above circuit by putting a full wave rectifier on the secondary of the transformer. That would give you approximately 24VDC, which is the alternate power supply recommended for the circuit. A note of CAUTION - whether or not 24VDC is appropriate depends on the rating of your motor. This takes us back to the first paragraph where I said we need to know the motor ratings.