|Assuming these are ppt files that you are opening, and assuming they are not password protected for modifying, and assuming they were not "packaged for CD", try this:|
Open any Windows folder
Click on Tools...Folder Options
Click on the File Types tab
Scroll down to the PPT filetype
It should say Microsoft Office PowerPoint under "Opens with:"
If it doesn't, make it so. If it does, I have no other suggestions.