I followed the Tutorial at: http://mediawiki.blender.org/index.php/Manual/PartV/Buffer_Shadows
for volumetric lighting, everything works fine, but since I got blender 2.4 instead of 2.37 which this despcription ist made for, I can’t find the “Halo step” slider, to enable volumetric shadow as well.
I found it, but I additionally had to enable Buffered Shadows to see the button. I always thought, that buffered shadows are a time-saving reduction of realism. Is it actually the other way round?