You can password protect the file from modification.
If you are using PowerPoint 2002/XP and above,
Go to Tools > Options > Security. Set your password there.
If you are using PowerPoint 2000 and below, this feature isn't supported.
You can though, embed your presentation into MS Word or Excel.
Check this out for more information:
"Password protect a presentation" http://pptfaq.com/FAQ00038.htm
--
Shawn Toh (tohlz)
Microsoft MVP PowerPoint
Site Updated: August 08, 2007
(All amazing PowerPoint skills here <g>) http://pptheaven.mvps.org
PowerPoint Heaven - The Power to Animate