Wallpaper Changer is a freeware application which is a wallpaper manager that can change your background images on every startup of Windows, once a day or at regular intervals specified by you. It features JPEG, PNG, GIF and BMP support and it has several options are provided, including the ability to change your images in any random order. It has the option to automatically tile, center or resize the selected image to full screen.
Wallpaper Changer has the ability to analyze the pictures and will determinate the best matching background and icon caption color which is readable for every wallpaper you apply to your desktop. In the options you can set the time for wallpaper change and also set the wallpaper to change at every Windows start up or every time you login to the system. There are also pretty good options to resize the wallpapers.
Wallpaper Changer also provides options to save your image list, so that you can load it back at any time. The application also supports drag and drop to the interface by which you can add the directories.
here is another free wallpaper changer (WPTimer), you can easily set the time interval in minutes: http://www.dreamscene.org/download.php#wptimer works great on XP and Vista 😉