Here is a comment emailed to me by bigbadsteve
It's a long time after your query, but nonetheless the following is I suspect the *cause* of the
issue [which no one has yet addressed in subsequent posts on the board topic]:
"If you use Windows Media Player to rip CDs to your computer in the Windows Media Audio (.wma)
format and then decide to give iTunes a try, beware! iTunes will convert those songs into its
Advanced Audio Coding (AAC) format, resulting in duplicate files for every song iTunes manages. To
avoid that, use a single music format (such as .mp3) that all media players can handle."
I'm guessing that the original unconverted music files would not have been be deleted by iTunes
(though you may have deleted them yourself), and that therefore finding out where all the music
files on your hard disk(s) are situated and developing a good backup procedure would prevent a
similar occurrence in future.
Uninstalling iTunes [if it was in fact the culprit] and not using it at all is of course an
Avoidance is better than cure, since some audio quality is lost ever time a sound file is converted.