Giana, there are folks that can help you with this-the sudden, without warning closing to the background screen thing. -today, my Poser 5 did exactly the same thing when I tried to render one figure and three lights.  My problem was the hair, two bangs (either one) caused Poser to simply close leaving me with my opening screen.  I don't know the reason, but in my case sometimes a given .pz3 file and say a hair piece will do this, yet on another .pz3 file, everything worked fine.  Plutom