You probably use a single picture as your Windows desktop background, ie your wallpaper. And you probably change that picture occasionally, to add a bit of variety to your computing. But if you'd like to have Windows change the picture for you automatically on a regular basis, you can do that too. What you need is something called a wallpaper slideshow, or desktop background slideshow. There are two easy ways to do this, and they're both free.
The first method is to download and install a free wallpaper changer utility. There are quite a few available, and you can read a selection of reviews and discussions at http://www.techsupportalert.com/content/best-free-wallpaper-changer.htm.
A recent recommendation in that list is Wallpaper SlideShow LT, which you can download at http://www.gphotoshow.com/wallpaperss.htm. It's free, runs on all recent versions of Windows, and is a 1.3 MB download. All you need to do is download and install it, point it at 2 or more of your photos, and then set the options how you like them. You can specify how often the image changes, or can configure it to change the image each time you start up your PC. You can even have a different image on each screen of a multi-monitor setup.
The other method is to use the Desktop Background Slideshow facility which is built into Windows 7. You don't need to download or install anything in this case, but you will of course need Win7. You can find out how to use the feature at http://windows.microsoft.com/en-US/windows7/create-a-desktop-background-slide-show. However, be aware that Wallpaper Slideshow LT offers more features than the built-in facility, such as not forcing you to store all your selected images in the same folder.
My thanks to reader Alin Brunetu for telling me about Wallpeper Slideshow LT.