Open up the Power Options Control Panel. If you're on a laptop, this can easily be done by right clicking on the battery icon.
You now have two options. You can change one of the existing plans to "Never" shut down the display or system, or create a new power plan - call it "Presentation Mode".
Check your current power plan by clicking the Change Plan Settings link. Change the settings accordingly. On a desktop, there's a single set of settings, on laptops there are two sets - one for battery operation and the other for AC adapter running. Make sure you change both sets for Presentation.