DO NOT USE A HOME MADE PLUG FROM THE WALL TO RUN YOUR COMPUTER FAN.
Not trying to call you out brimstone, but this is one of the worst ideas ever.
AGAIN, DO NOT DO THIS, YOU WILL EITHER A) CATCH YOUR HOUSE ON FIRE or B) SPIN THE FAN SO FAST IT FLIES OUT AND DESTROYS SOMETHING. BOTH ARE LIKELY.
You can actually buy a transformer using the correct voltage and wire the fan into that.