Windows 7 now comes with some new features for its Desktop Wallpapers, including an option to create a slideshow. You can choose any of the default Windows wallpapers or you can create a slideshow with your own images and you can choose as many as you like.
To do this, simply right-click on an empty spot on the Desktop and select 'Personalize'. Click on the 'Desktop Background' option then click on the 'Browse' button to select the folder your images are located in. Hold down the 'Ctrl' key and click on all images you want to include in the slideshow. In the 'Change picture every:' drop down menu, select how often you want the images to change, click on the 'Shuffle' checkbox to randomize the images, then click on the 'Save Changes' button to apply your changes.