"Create a default "blank" presentation with your own defaults"
http://pptfaq.com/FAQ00245.htm

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Shawn Toh (tohlz)
Microsoft MVP PowerPoint
Site Updated: May 19, 2007
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> Most of my powerpoint usage is in the portrait format so I would like to
> change the default so that anytime I open a powerpoint slide it is in that
> format. Anyone have an idea if this can be done and if so how?