There are many applications that can create video slideshows. Here are some solutions:

Windows

You can use the free program Movie Maker, which comes with Windows XP (Service Pack 2) and Vista. Here is a very basic tutorial for Movie Maker 2. A more complete description of its features can be found here. Answers to Frequently Asked Questions about Movie Maker 2 are here.

Another free program you can use is Photo Story 3, which can be downloaded from Microsoft. Among other things, this allows you to add panning and zooming effects to your still photos. (There is no specific Vista version of this software, but the XP version works with Vista as well.)

A very useful resource for those using Movie Maker and Photo Story is the Windows Movie Makers Website.

If you have PowerPoint, you you can save your presentation slides as jpg files, which you can later transform into a video by adding sound with Movie Maker. This, however, will not preserve animations and special effects. A tutorial showing how to convert your PowerPoint presentation into a movie is here. Alternatively, you can use screen capture software to capture the presentation and save it in video format. Free screen capture software is here. Commercial solutions, which allow you to edit the screen capture, include SnagIt (suitable for presentations that do not include video clips), Camtasia, and Captivate. There is also a free add-on called Microsoft Producer that can transform your presentation into HTML pages with embedded WMV video. (Unfortunately, this works with Power Point 2003, but not with Power Point 2007.) 

Mac OS X

All new Macs come with the iLife suite, which includes iMovie. You can use this to create your video slideshows.

Here are tutorials for iMovie 06, iMovie 08, and iMovie 09. Of course, the tutorials talk about making a movie using video clips. However, you will be making a slideshow using still photos. This is actually easier. You will just put pictures where others put video clips. To do that in iMovie 08 or 09, click on the Photo Browser button. This button bears the picture of a camera and is located on the right side of the silver-colored toolbar that runs through the center of your screen. After you click this button, you will see thumbnails your pictures. Just drag the ones you want to the Project window. To do the same thing in iMovie 06, click on the Media button, then on the photos tab. Navigate to your photos and drag the thumbnails of the pictures you want to the window on the left.

If you have PowerPoint or Keynote, you can create your slideshow with it, export the slides as jpg files, import them into iMovie, and add narration. Alternatively, you can save the whole presentation as a QuickTime movie, import it into iMovie, and add your narration.

In theory, it is possible to record the narration directly into PowerPoint and Keynote before saving the presentation as a QuickTime movie. That way, you could avoid using iMovie altogether. However, please test whether this procedure works with your system. It may not. Quicktime 7.5 installed, for example, reportedly fails to import the sound embedded in Keynote presentations.

Note on versions of iMovie

The latest version, iMovie 09, does not have a traditional timeline view. However, you can use the "Precision Editor." 

iMovie 08 had neither the traditional timeline nor the "Precision Editor" and lacked so many functions that a number of people were unhappy with it. To know more about the limitations of iMovie 08 you can read this review. Nevertheless, these limitations should not be a concern to you if all you are doing is editing simple video slideshows.

iMovie 06 had a traditional timeline view. Until January 2009, if you had iMovie 08 and you preferred iMovie 06, you could download iMovie 06 for free  from Apple. However, this is no longer possible.