Desktop Movie is a simple application for playing a movie on your desktop (replacing the regular background).
The first time you run Desktop Movie on Windows 8, it warns you about "patching the Registry", but this isn't as stupid or scary as it sounds - it just adds an entirely legitimate Registry key to improve compatibility. Readme.txt tells you more.
The program interface is basic. Launch the single executable, choose a video file (anything supported by DirectShow should work) and it'll start to play.
There's no fancy player bar which appears when you hover your mouse somewhere, unfortunately. Instead playback is clumsily controlled from a system tray icon and a separate dialog.
Still, it does work, and if you just want to play a single video non-stop then it's easy enough to use. Drag and drop a file onto Desktop Movie, or maybe set up a shortcut which passes the file name on the command line ("dmovie.exe video.mp4") and the program will handle everything else.
Desktop Movie's basic interface means it's not a good choice as a general video player, but if you'd just like to play a full movie on your desktop then it's a small and simple answer.