Why would you want to install Poser 5 over PoserProPak?
You would be better off installing Poser 5 in its own separate
directory and then linking Poser 5 to your ProPak library.

To do this, once you install and open Poser 5, got to the right of the
screen and keep clicking the libraries up arrow until you get to the root directory.
You should see Poser 5 and Downloads folders.
Then click on the plus sign at the bottom right of the libraries list.
 The pop-up window will then let you link to the ProPak library runtime.