This codec pac is excellent for XP
http://www.xpcodecpack.com/
Or even easier download the VLC player it plays just about all file types and has all the codecs pre-installed...If VLC doesn't play it there's something wrong with the file...
http://www.videolan.org/vlc/
To make VLC your defrault player....
Go to your shared folder (outside of Limewire) you will find this folder by clicking on one of your files in your LW library and clicking explore...
Right click on one of your movie files
Select open with
Select choose program
select VLC
and tick the box that says always use this program to open this file type
Select OK