PowerPoint presentations are based on *.pot files which
are referred to as design templates. These templates are
then used to create the "master sets" from which you then
build your presentation. Now, the only way I could see to
change the default font would be to edit the design
template on which your presentation is going to be based
i.e. edit the master. NB the default design template for
new presentations is the one saved as "blank.pot".
