VLC is a swiss army knife media player. It plays practically any digital local and streaming media, including Youtube vids. All codecs are internal so no other dependencies. Sure its GUI isn't as pretty as Windows Media Player. Still the best on ANY platform while continously being updated for XP.
I have also used MPC-HC which relies on external codec packs. Unfortunately MPC-HC is officially dead so your best choice in XP would be VLC.