There has been a tremendous amount of dissatisfaction about how Vista handles any version of Poser installed into it's Program Files folder.  If Poser resides in mentioned location, Vista will not allow easy write-to unless UAC is inactivated.  This might be your issue, but I'm just speculating.  The recommended fix is either turn off UAC completely (thefixer has posted instructions how to accomplish this.  I don't use Vista, can't help there) or relocate Poser into it's own folder on your main HD.  Again, I am only speculating; perhaps some one else could elaborate further towards a solution.