In order to change the default font for PowerPoint 2007, we have to create a base
template and save it in default templates folder. Please follow the steps mentioned
below to create and save a base template and see if that will meet your
requirements.
1. Start PowerPoint 2007.
2. Select View tab ; then select Slide Master
3. On the Slide Master (Slide Master: the slide that stores information about the design template applied, including font styles, placeholder sizes and positions, background design, and color schemes.), make the changes that you want to be
reflected in every new presentation. You have to make the changes to each slide of the slide master.
4. Then click Close Master View.
5. Click Office Button and then click Save As > Other formats
6. Pick PowerPoint Template from the Save As Type list box
7. Save the template as Blank.potx. PowerPoint will automatically chooses the folder as %Appdata%\Microsoft\Templates, so accept the default location. The default location would be
9. Close the file and start a new presentation to verify the font.
For your testing, attached a sample Blank.potx file based on your requirement.
(Courtesy of: Office Deployment Support Team Blog at http://blogs.technet.com/odsupport/archive/2009/04/24/how-to-deploy-specific-fonts-in-office-2007.aspx)