I can't offer you any hands on advice, since I'm using ME, but what I have seen mostly good comments regarding Poser and XP. If I had to guess, I would say it had something to do with software that was installed improperly (were logged in as administrator when you installed it?) Did you delete the old installation folder and restart before attempting to reinstall? I know that there are files that store information about the preferred state, and they can become messed up.