Under Runtime:prefs you'll see LibraryPrefs.xml, open this in a text editor, and the fourth line down will have previewNum "XXX" (I think default is 100), chance the number to anything below 1,000 and save, I've got mine set to 200 in Poser 6, which should be more than enough.

Make sure you don't have Poser 5 open when you do this, or the changes won't be saved.

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