This is usually caused by PowerPoint changing the screen resolution when you go live with the slide show. Make the following setting changes to PowerPoint to correct the problem.

1. Open the Microsoft PowerPoint program. 
2. Click the Slide Show Tab. 
3. For office 2007 & 2010 Use the resolution dropdown on the toolbar to change the resolution to Use Current Resolution.
   
4. For newer versions of office, click the SetUp Slideshow button, and change the resolution setting to Use current resolution at the bottom right of the window that appears, then click OK.