This is a much abbreviated version from an email:
Convert 4:3 powerpoint presentations to 16:9 (wide-screen)
Keynote (apple) can resolve this problem.
1) Open the original presentation in Apple keynote.
2) Open the inspector, choose document >slide size Choose the new size
3) Export the new presentation from Keynote to Powerpoint.
Workaround from Microsoft:
http://office.microsoft.com/en-us/powerpoint/HA011420901033.aspx