a computer fan takes verry little power to run if you give it to much it will burn up the motor i would get one of those adjustable output power cords
that they make for hand held cd players and stuff like that at wal-mart in the electronics section there about $8.00 and go 3 to 12 volts then just cut and splice start with it on the lowest setting and move it up until the fan starts working dont over do it though the fan might even say how many volts it takes right on it

good luck