Not sure if it would help, but do you have the Windows Media Player Codec Pack installed as well? Upgrading to Windows Media Player 10 may be something worth trying as well. Usually the only time I have an MP3 working in one program but not another is because it was encoded in a way that the specific program didn't support. Otherwise I can't think of anything else but to uninstall and reinstall all your codecs

edit: you may also want to update your sound card drivers, and disable/enable the hardware mpeg encoder if you have a sound card built into your motherboard.