Solution (1)
Take an x-box usb port and connect it to a 5V relay coil with a small diode (1N4148) cathode of diode to +5V USB, anode of 1N4184 diode to ground and both leads of the diode across the relay coil. Connect a discarded 12-volt wall wort through the relay contacts to power the fan.
Solution(2)
Take the +5V wire from a x-box USB port and apply it to the gate of N-chan power MOSFET, connect a 10k resistor gate to ground to keep the MOSFET turned off when the XBox is unplugged. Take a 12v wall wort and connect the - lead to the common USB ground. Connect the +lead of the wallwort to the + wire of the fan, Ground the source of the MOSFET to the USB ground. Connect the - wire of the fan to the Drain terminal of the MOSFET.
Solution(3) substitute a 1Amp medium power NPN transistor to replace the n-channel MOSFET. Connect the +5V of the USB port to the base of the transistor with a 1K resistor in series with the base. Ground the emitter of the NPN transistor to the common ground of the 12V wall wart and the USB. Connect the -lead of the fan to the collector of the NPN transistor, As before, connect the +lead of the fan to the +12V output of the wall wart.
Solution(4) use a TI boost convertor IC to convert the 5V power from the USB +5 wire from the X-box to +12V to directly drive the fan. Some, but not all, PC power supply fans require just .125 amp at 12V and this equates to a current draw from the USB port at 5V which might be within the power limits of the X-Box USB port for powering a device operation. See attached.
(schematic upon request)
Solution(5) hack into your X-box and find a 12V power supply point and bring it out to supply the cooling 12V fan. If your X-Box has a DVD/CDROM drive, it has a 12V supply powering it, so easy to locate this power point.