Windows XP originally shipped with Windows Media Player (WMP) 8 in 2001. It has been updated several times since then to the highest version available for XP, which is WMP 11. You may find, however that certain audio or video files, DVDs in particular, may not play at all or if they do they may not play well. They may "stutter" or be out of sync. Some issues might be related to your connection speed if you're streaming video; other problems might be related to your graphics card. If you have ruled these two factors out and you are still having trouble it might be related to what are called "codecs". Codecs are able to code and decode (interpret) the audio and video for various types of media files.
To solve the playback problem with XP's WMP, first be sure that you have the latest version of Windows Media Player installed. After that you can visit a website called codecguide.com http://codecguide.com/about_kl.htm
to see what is available for download. When solving playback issues for the school where I work I don't have any time to "guess and check" so I have used the K-Lite mega pack to cover all bases and it has so far done the trick for solving any of our DVD playback troubles.