What TAJ said and...
Make sure you have installed the latest service pack for PowerPoint
2003. When PowerPoint 2003 was first released, it had trouble opening a
variety of PowerPoint files (including, sometimes, ones created by
PowerPoint 2003). This was quickly fixed with a service pack, but there
were a lot of PowerPoint 2003 applications already out there that needed
updating. Open PowerPoint and go to the Help menu and choose Check for
Updates. Follow the instructions if updates are required.
--David