To use the use the left mouse button to move forward one slide and use the right mouse button to move backward one slide, do this:

• Start PowerPoint.
• Go to Tools | Options.
• Click the View tab .
• Deselect the check box labeled "Popup menu on right mouse click".
• Click OK.

Press F5 to run the slide show and you'll find that a left click moves to the next slide, and a right click moves it back one slide.