for blender, are you using windows?, if so, blender usually has all the python it needs for the scripts that come with it, but sometimes not, my advise, unistall blender, install python 2.4, then install blender 2.36, that should do it. for yafray, I had the same problem with a lot of my programs that are meant to run in a shell, if in windows xp, right click it, select properties, then set a compatibility mode for win95, that should work, if not, talk to someone with a bigger brain or go to the elysiun forums.