The only way I could do this in earlier versions was to set the external monitor as another desktop and run two versions of the power point presentation. The slideshow on the external monitor and notes on the LCD screen.
Powerpoint 2007 makes this easier
PowerPoint 2007 has a feature known as Presenter View that allows you to run your presentation from one monitor while your audience views it on a second monitor. You can view thumbnails of the so you know what is coming up next and also view your Speaker's notes so they can be used to assist you in your presentation, all while your audience sees only the presentation. To utilize Presenter view, you will need to turn on multiple monitor support on your computer or laptop. To deliver a presentation on two monitors by using Presenter view, follow these steps 1. Open the desired PowerPoint presentation.
Select the Slide Show tab.
Go to the Set Up group and select Set Up Slide Show.
Choose the options that you will need from the Set Up Show dialog box.
Under the Multiple monitors section, select which monitor you wish the presentation to be displayed on and check the Show Presenter View checkbox.
To begin the presentation:
Select the View tab.
Go to the Presentation Views group and select Slide Show.