All the rogue procesess that could be using the file normally have incomprehnsible names (Think Shimranese)
You might try qttask (quicktime task manager) or anything with "real" in the name (They are for REAL player) as they would likely open movies.
Also, if you got the movie file via P2P or bittorrent, you should check to see if anyone is downloading it, from you, and if they are, cancel the download.