To do it via the keyboard, in PowerPoint select
File,
Save As and then select the
Save as Type:
Windows Media Video (*.wmv)
To do it through VBA, use the
Export method in the
Presentation object.
See
Presentation.Export Method (PowerPoint)[
^]
This VBScript[
^] example (found using Google search) showing how to control PowerPoint could be used as a starting point to help you develop your solution.