It seems that Microsoft has built functionality into PowerPoint to disable the screensaver during a presentation. I've found the following in their knowledge base:
Microsoft realizes that users frequently give presentations from their laptops. With PowerPoint 2000, Windows screensavers are disabled during a presentation. In addition, the laptops low-power screen mode has been disengaged to prevent interruptions during a presentation.
Question: For security purposes, I would NOT want PowerPoint to disable the screensaver. Is there a way to prevent this from happening?