First you need to ask yourself why you want to protect your PowerPoint Presentation. Are you planning to protect secret information contained in the presentation-because if you plan to show the presentation to anyone, once they see the information your secret is out. If you are aiming to protect the actual presentation itself, then you are in luck. You can protect your PowerPoint from being copying (or altered in some embarrassing manner).
Microsoft PowerPoint versions later than 2002 (XP) have the option to password protect your presentation. These can be added by choosing Tools from the menu at the top of the screen. Under tools select options, and then click on Security. You can create a password to open the presentation or a password to modify the presentation.
Probably the next best option you have is to zip it. Zipping it up will create a self-extracting EXE file that can be protected with a password. If someone attempts to open it, they will be prompted for a password. Without the correct password they will be denied access to any files within.
The easiest (but least secure) method to protect your presentation (this one does not involve a password) is to rename your presentation from a .PPT to a .PPS file. This will immediately open the PowerPoint and start the slide show not allowing any modifications. However, the experienced computer user will recognize that they can start up PowerPoint and then open up your file to modify it. This will probably protect against small children, but not against another business or serious competitors.