• Rename the PPTX to give it a .ZIP extension. For example, rename TEST.PPTX to TEST.PPTX.ZIP.
• Open the ZIP file.
• Open the ppt folder within the ZIP.
• Copy (drag-and-drop) presentation.xml from the ppt folder to your desktop.
• Edit it with a text editor:
• Find this string:

<p:sldSz cx="9144000" cy="6858000" type="screen4x3"/>

• Change it to:

<p:sldSz cx="12191996" cy="6858000" type="screen4x3"/>

• Save presentation.xml.
• Copy it back into the ppt folder, replacing the existing file.
• Close the ZIP file and rename it back to TEST.PPTX.
• Open TEST.PPTX in PowerPoint.
• You'll have to move images left/right to get them where you want them.

From http://tumblr.cgeier.at/post/43296834116/convert-a-4-3-ppt-slide-deck-into-a-16-9-aspect-ratio and http://www.pptfaq.com/FAQ00566_Make_screenshow_fill_a_wide_screen_display.htm