Windows Dreamscene is a tool that comes bundled with Windows Vista Ultimate edition, allowing users to set a video as a desktop background. Free video content has been provided over the Windows Update service, allowing selections of several nature videos.
This tool does not use as much CPU power as you may think, as some video rendering is performed by the graphics card (GPU) and is automatically paused when not in view (during tasks such as gaming). This substantially reduces the processing load in the majority of situations.
To use Windows Dreamscene you must have the Ultimate edition of Vista. Assuming this is the case, right click on the desktop and select Personalization.
Now, click Desktop Background.
From this window, choose Windows Dreamscene Content from the dropdown menu.
On this screen you can select the video you wish to play as your desktop background and then click OK once the choice has been made.