Well, unless you implement some sort of DRM (Digital rights management), your "authenticated" users will be able to download/forward your videos. There are tons of tools to record streaming videos by a click of a button...
google for stream capture tools, and tons of it...
DRM's are usually expensive, plz look for any cheap DRM's, I found a thread in stackoverflow featuring a free DRM, didnt test it, plz have a look on it also.
Is there an open-source DRM solution?
Another easy way would be to, store your files in one location, and you could generate some unique ID, which would get expired after a pre-set duration, or after x number of times, and you could use some programming logic to send the content of the video files, upon client submitting the unique ID (TransmitFile in C#)