The problem might be that you've got Poser installed on an external harddrive. I had a similar set-up at one point, and had the same problem opening pz3 files without doing File>Open in Poser. It seems Windows doesn't appropriately recognize or accept Poser as a valid program for opening its files when the program is on an external drive. I would suggest, if you've got space on your main HD, installing Poser there, and trying to fixe the association and open your pz3 file outside of Poser.

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If you're joking that's just cruel, but if you're being sarcastic, that's even worse.