when opening Poser 5, I get this error message, and then Crash to Desktop.

This is a known error in earlier builds of Poser 5. It has since been patched. Do you have the latest service release installed? If not, you can download the application and figure updates at Curious Labs.

it seems the feet are always "rooted" to the "ground" if you could call it that ....

You probably have IK (Inverse Kinematics) enabled for the legs. You can disable IK from the Figure menu.