If the fan is connected directly to the PSU (e.g. with a Molex connector like used for hard drives) then the only way to have it selectable would be to have a physical switch somewhere. If it connects to the system mainboard, you could find out whether the fan header it goes to is controllable in the BIOS setup. If it is, in one of the BIOS menus there will be an option called something like Case Fan, or whatever the header is called, and a choice of speed/enable/disable, or sometimes a temperature selection.
Edited by Platypus, 16 November 2015 - 11:01 PM.