Eliminating Light Leaks

To attenuate the leaking of outcoming light through edges inside :
Go to the light's Detail tab and in Falloff select :
Inverse Square (Psysically Acurate)
You can also add a little Ambiant Occlusion FX.
Reduce the amount of Ambiant Occlusion…
Or else, you can duplicate the same light
and choose Raytrace hard (for the duplicate)
and attenuate the Color and percentage in the Shadow tab.
Using both lights with different shadows, you'll have to
adapt the intensity of both lights.
If you select PAL/NTSC in the Ambiant Occlusion tab
you also get a softer blurr (more obvious around the window)