Make sure when you install Poser 8 you use the option in the beginning for Vista UAC.  Also a friend had problems with her firewall blocking and not allowing certain things to run.  I do not remember what her firewall was, but it is something you might want to check.

I have installed Poser 8 with Vista UAC and no problems with that part.  The only two problems I am having is no textures or mat files are loading with either pz2s or cr2s or props.  Still trying to figure that one out. Have  ticket into Smith Micro but they are baffled as well. Hmmmm Also when loading more than one figure it wants to crash on me.

Am sure someone with more experience can help you better, just things I have found with mine.:)