There is a little known Windows XP PowerToy called the 'Wallpaper Changer'.
This PowerToy changes the background wallpaper of your computer's desktop
automatically from images you select at predetermined intervals
(based on: minutes, hours, or days).

After the program is installed, it loads when you log into your computer, and sits in the
notification area. To change its settings, all you have to do is right-click the icon.