I'm not satisfied with the quality of video playback in Unity (Movie texture which I run after converting video with Miro converter and importing it to Unity),... so, is there any chance that there is a 3rd party library which I can integrate inside Unity to play my movie file? My movie is full HD intro movie to my 3d app which can be played from application menu. I've tried to use Media Player Classic (MPC) as external application, but the problem is, when I run MPC as new Process from application menu, my menu minimizes and this looks awkward for one stand alone app. When intro finishes, I have to maximize my menu by clicking on application icon placed on Windows taskbar.
There is a C# wrapper for the well-known and open-source FFmpeg: http://www.ffmpeg-csharp.com/
Unfortunately I don't know what level of freedom you have in selecting external libraries for use in Unity projects, but you might want to look at either:
Both of these options may very well be impractical due to the level of control Unity may give you over the lower-level OS objects -- window handles, primarily, and hooks for periodic updates.
Even if you got either of the above options to work, you'd be sacrificing portability (if that matters).
Finally, you may want to browse some of the resources linked in this external Unity resources question, which might lead you to something useful for your purposes.