DreamScene allowed you to set a video as your desktop wallpaper in Windows Vista. The following hack allows you to re-enable it in Windows 7:

1. Download Windows7-DreamScene.exe (mirror). An alternative called Windows7-DreamScene-Enable.exe which doesn’t require you to have Admin Rights.  
2. Run Windows7-DreamScene.exe as Administrator. It’ll copy DreamScene.dll to %WinDir%\System32 folder, DreamScene.dll.mui to %WinDir%\System32\en-US folder and add the required registry keys and values. Then it will then automatically restart Explorer.
3. The installer also creates a Windows DreamScene folder in %WinDir%\Web\folder. You can place any video file that you want to set as DreamScene background wallpaper here.
4. To set a video clip (.WMV or .MPG) as your background, right click on the file and choose “Set as Desktop Background” from the context menu. That’s it!

Source:→ MyDigitalLife