Some time ago (about 10-15 days) I asked how I would change the
default font in PowerPoint from Ariel to Comic Sans MS.
PowerPoint doesn't have a global default font; each presentation or template
has a default, though.
To set the default for the current presentation, click off the slide so that
nothing is selected, then choose the font and size you want on the formatting
toolbar.
Any new text boxes you create will be in that font.
Text in placeholders (the "Click to add xxx" boxes) is controlled by the Slide
and possibly Title masters. You'll need to use View, Master, Slide Master to
edit those.
Once you have a presentation set up, you may want to save it as a template so
you can base new presentations on it and not have to repeat all those steps.
Save a copy of the presentation, delete all the slides from it then choose
File, Save As, choose Presentation Design (*.POT) and save the file.