Switched 5V Power Source XBox 360
As Jasen commented, you can put some LEDs in series and run them from your 12V Fan supply.

E.g. 5 x 2.2V LEDs in series = 11V so you need to drop 1V at say 15mA so the current limiting resistor would be 1/0.015 = 66 ohms or thereabouts.







simulate this circuit &ndash; Schematic created using CircuitLab

The idea in Chupacrabras' comment would be to use the 12V fan supply to turn on the LEDS while powering them from 5V. I think that might look something like this (untested)







simulate this circuit

The above values should be OK for small indicator LEDs with a forward voltage of 2.2V and a working current of 10-20mA. If you are planning to floodlight an arena from your XBox, you'd have to respecify and recalc.