If there aren't many slides, you can try this out.
Manually add a black AutoShape(square) that covers the entire slide. Do it
on one slide first.
Then, click Slide Show > Custom Animation.
Click Add effect > Exit > Disappear.
Set the "Start" to "After Previous", then have it place as the first effect.
By doing so, the black autoshape will disappear upon the start of slideshow.
Copy and paste the black autoshape to all the slides. Make sure the
autoshape is the first to be animated in every slide.
Now, set a password to the presentation to prevent modification (Tools >
Options > Security). Save your presentation. Without the password, users will
only be able to open the presentation as a read only, which doesn't allow
them to move the black autoshape.
--
Microsoft Most Valuable Professional (MVP PowerPoint)
Site Updated: April 13, 2006
Added new portfolio
http://pptheaven.mvps.org
PowerPoint Heaven - The Power to Animate