Vista doesn't have anything to say when you start XP setup since Vista isn't loaded yet. Did you by any chance inserted XP install disc and then ran it from within Vista? If so it's logical it wouldn't let you. XP setup can't downgrade newer OS, same as Windows 98 can't downgrade XP or 2000. Only format and clean install.

I just tried Poser 6 with Vista and it works normally, as does Poser 7.