What exactly are you tring to do ? Play mp3's ? Up-grade to or re-install
WMP9. Do you get error messages, if so what are they (be as precise as
possible) ?
If you're trying to convert into mp3 format, make sure you have a decent
encoder lie Lame, or Blade (search the web and you will find them). Or
Goldwave will do the job nicely ( http://www.goldwave.com ).