Animation on desktop is different with every Windows operating system. The easiest way to have an animated background is to use Dreamscenes. Dreamscene technology allows you to have a video, image, animated images as your favorite background. There are two different approaches to the problem based on the version of your Windows system like:
Windows XP: To have an animated background you will need a software like XPScene which allows you to set up a dreamscene in few easy steps. Download the software package, install it using the on-screen instructions and then start the application from your desktop. Next chose your favorite video, image, animation using the first field then the output folder. After conversion is done right click the HTML file you have and set it as Active Desktop file.
Windows 7: This way is the easiest because of the default support of Dreamscenes in Windows 7. You will need to run DreamScene Activator to enable this feature then right click on your video, image file and set it as your animated background.