Missing Codec & Avi Preview

When I do Avi Preview on an XVID movie, the screen always FLICKERS.
Same thing when I try to play XVID movie via BS Player.
When I use WMP, no prob. Am I missing an XVID codec for AVI Preview & BS Player ? Where can I get this specific codec ? Is it safe to d/l a codec pack over a pack I currently have ? I dont want to screw anything up ?
Thanks

have you tried Videolan, and have you set up correct codecs through KLite codec pack? Videolan can also be set up to use for previewing.