Welcome to the forums.
Generally, it doesn't make a difference to any media player's operation to have other media player's installed on your system. (I tend to leave WMP as it comes and use other players around it, such as VLC and Clementine or iTunes etc.)
Are you only having the issue with one film? Have you tried playing it in a different player? (i.e if its not working in VLC, have you tried WMP, MPlayer, Quicktime, Banshee... the list goes on)