Windows Vista and Windows 7 has a feature to automatically change the wallpaper. But you can install Microsoft Powertoy – Wallpaper Changer to give the same functionality of automatically change the wallpapers in Windows XP too. Here is how :
Download the Microsoft Powertoy Wallpaper Changer from http://www.microsoft.com/downloads/. The download page requires Genuine Windows validation. You can also download the file from the direct download link : powertoys_wpchanger.exe.
Run the setup and after installation you would see the Wallpaper Changer Options window. In this window you can choose many options.
You can choose the folder containing all the wallpaper images in the folder tree view on the top-left. You can choose the the time interval for changing the wallpaper (the default is half an hour).
If you want to revert back to just one wallpaper, you can choose the option Show One Still Image. This will stop the automatic changing of wallpapers.
You can also right-click on the Wallpaper Changer icon in the system tray to get the options menu.
You can choose to turn off or on the automatic wallpaper changing. You can manually change the wallpaper to the next image in your selected folder. You can view a fullscreen sildeshow for all the images in your selected folder.