Use the compositor Glare node with the type set to 'Fog Glow'.
Ensure you have a suitable emission material behind the 'windows' to ensure there is a bright area for the glow to act on. Add a 'Glow' node and set the type to 'Fog Glow'.
Adjust the parameters of the Fog Glow and/or change the intensity of the emitter to get the desired effect.
No glow, a moderate glow, a strong glow :