I think there is some misunderstanding here.
What you should do under Vista and Windows is NOT to install the Runtime in "Program Files (86)".
You can happily install Poser itself to "Program Files (86)" which is the default for the Poser install program.
The default for the runtime in Poser 8 is (I believe) to install to /users/public/documents - which is perfectly fine.

In the install program there is some compatility option to install the Runtime to Program Files and that is what you should avoid in Vista and WIndows 7

I would recommend to install Windows 7 64 bit. Even Poser 8 will profit from it since there is more memory available for the application (4GB instead of 2GB in a 32 bit Windows)