download a XviD codec by clicking on the blue underlined word. And then it will play in any player. Or try using Divx player. That works great
Depends on the container. If it is an avi, then it will play in any dshow based player, at least the video will. Xvid is just a video codec. Personally I like mplayer, supports Xvid decoding as well as avi, mp4, mkv, ogm, nut, etc. containers along with most audio formats (mp3, AC3, DTS, aac, vorbis, etc.).