for playing as well as managing music files, i would recommend foobar2000. it does not eat up system resources and effectively plays and manages all types of audio files.
for video, the KMPlayer is the most lightweight option.
if you want to edit movies, just go to a store and buy Adobe Premier Elements 4.0. It costs about $60 but is completely worth it.
Edit: VLC is good, but it only serves as a player. It does not even attempt to manage your media library.
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