You could also use Webshots. You can set the cycle to change pictures every second up to 120 seconds and you can use it to change your wallpaper every 15 minutes. You can download it here .
As for using XP, right click on the desktop, go into Properties, and in the screensaver drop down box, choose My Pictures Slideshow. Use the settings to control how often you want the picture to cycle. Bruce
Todd, just right click on your desktop, click properties, then Screen Savers tab. From the drop down box, choose My Pictures Screensaver, and use the settings to choose the folder and other things regarding the screen saver.