This is how it is in PowerPoint 2002.
Follow these steps:-
- Create your presentation
- Click on File\Save As
- Type in a filename and then click on the tools button at the top
- Click on Security Options...
You can now specify a password to open the file and a separate password to modify the file.
If you want anyone to view it but no-one to modify it then leave the "Password to open" box clear and then enter a password in the "Password to modify" box.
When someone opens the file it asks for a password to modify the file. They then have the option to enter a password or to open it read only.
This should be what you're looking for.
Hope this helps