You can password protect it if you have PowerPoint 2002 or 2003. With older
versions try this.
Save your presentation (the one you want to protect) as a PowerPoint Show
(.pps). Open a New presentation. Insert the .pps file you saved previously,
as an object (Insert > Object > From File). Resize the image you just
inserted to cover the screen. Save the new presentation your just created as
a PowerPoint Show (.pps).
Let me know how this works for you.
Sandy