What you could do or try, if its possible in the scene you are using, is to render the image two times. First you render it normally with no lights in the windows. The second one you apply a 100% black material to all non lighting windows and turn off all lights in the scene. To the windows you apply a 100% white ambient light, that way you should be able to get a mask. Then you load both the images into PS or Gimp or whatever you prefer and use the mask to select the windows and then a apply the light here, with some glow (blur) and so on I think you can make it work. Maybe do it in several layers so you can have different intensity for the lights so it looks more like the image above.