Your question has some conflicting statements so its difficult to determine what your exact issue is.
You state that the movie you are trying to watch is on a site, which suggests that you are watching a movie in a web browser - in this case you would not need the xvid codec, you would instead require the relevant plugins for your browser to play the movie.
If the movie file is on your computer, then you will require the xvid codec to watch the movie. If you tried to watch it without the codec, the player will usually display a black screen and just play the audio - this is because it does not have xvid to decompress the video portion of the movie, but has the mp3 to decompress the audio.
It does not matter which media player program you use to view a file. Any codecs you install will effect all installed programs.
You mentioned that you download xvid, but you didnt mention whether or not you installed it.
I recommend deleting this question and creating another Yahoo Question with the following details so that we can help you properly:
- where the movie file resides (on a website or on your computer)
- (if file is on your computer) what media player program you are using
- whether or not you went through the installation of xvid and what version of xvid you got
- where it tells you that you require xvid (does the media player program instruct you to do this or does the website, how do you know it is xvid that is missing)
With these details, we can help you quickly.