Causes
This may occur because an administrator has deliberately changed settings that prohibit opening/saving certain file types in PowerPoint, or because of a change made by Service Pack 3 (S3) for Office 2003 or Post-SP3 hotfixes.
First, if you have Office 2003 and have just noticed this change in behavior, it's probably because SP3 has been applied.
In that case see this:
http://support.microsoft.com/kb/938810
Information about certain file types that are blocked after you install Office 2003 Service Pack 3
The page talks about making changes to the Registry. This can be complicated and may cause other problems on your computer if you make mistakes. But scroll down and you'll find files "to re-enable" the disabled formats. You can download them from Microsoft's Download Center and run them to fix the problem.
If you have Office 2007 or have 2003 but haven't applied SP3, try this page instead:
http://support.microsoft.com/kb/922847/en-us
You receive an error message when you try to open or to save a file type that was blocked by your registry policy settings in PowerPoint 2007 or in PowerPoint 2003
In any case you cannot do the following in PowerPoint 2007:
• Save a presentation in the PowerPoint 95 or earlier file formats
• Open files saved in PowerPoint 95 or earlier PowerPoint formats
Resolution:
Safe in Presentation (*.ppt)
