Hi the best and most popular codec pack for the XP is K-Lite
Codec Pack. You can download newest version 4.95 of the package
from the http://www.video-codec-pack.com
Also would like to recommend you stable video player, so named
VLC video player Video Lan Client, goes together with some codecs
as well.
All this tools and packs you can download from the:
http://www.video-codec-pack.com