i don't understand why people keep titling their threads "Help!!!"?
anyhow, dear matrix1, you seem to have an
xy-problem:
you want to watch a video, it doesn't work, and from some "googling" you assume that installing xvidcore is the solution.
wrong.
which video?
you also don't tell us which distro you use, and no error output from your current media player. a crucial point for every thread you open here.
please read the first link in my signature.
most probably (and i mean, like 99.9%) the easiest solution is to use a different media player.
so, install something like mplayer, mpv or vlc and try with that. it should Just Work (tm).