If the video files work in your Windows Media Player, then it should work in PowerPoint, uninstall and reinstall PowerPoint should fix your problem as Add-ins (Drivers) required to play the file may be damaged, corrupt or missing.
Step 1. Download Freemake Video Converter from freemake.com
Step 2. Add the video files you wish to convert
Step 3. Convert them to WMV
Step 4. Test. Don't delete the converted files & Leave Freemake open.
If that doesn't work, proceed..
Step 5. Convert them to MPEG
Step 6. Test. Don't delete the converted files.
If that doesn't work, proceed.
Step 7. Move the presentation and video files to another computer.
Step 8. Test. If they work on the other computer, then your PowerPoint needs to be uninstalled, settings deleted and reinstalled.
If that doesn't work, and you can't get any of the files to work on either computer, then maybe the problem your having is affecting both computers. I'm sorry but I can't provide anymore help.
