A window operator mountable on the frame of window having a hinged movable sash including a drive assembly for positioning the movable sash and a separately installable drive shaft for operating the drive assembly. The drive assembly includes a drive chain coupled to the movable sash of the window that extends and retracts to position the movable sash. The drive assembly can be mounted to the frame first without the drive shaft. After the window is installed, the drive shaft can be then inserted into the drive assembly and used to extend and retract the drive chain and move the movable sash. The window operator includes a drive shaft retention assembly for receiving and retaining the drive shaft once the drive shaft is inserted into the drive assembly.