I can't watch any videos,also can't listen to any music,a windows shows that a windows codec is missing.my one is windows xp.please help me by answering this question.

Would need to download and install codecs for xp

Here is a link for codecs, download and install and install with default options it gives during install
XP Codec Pack - Download

Or you could download VLC player, which can play most formats of media

VLC media player - Open Source Multimedia Framework and Player

Also you might want to check for updates of codecs, which can be found at the vlc site.