Are you sure that the files in your iTunes library are .wma?? Right click on a file name > get info. You'll see the file format there.
Which version of iTunes are you using (to find out, in iTunes click on 'help' > about iTunes - the version number will show)?
ATM I'm back with iTunes 6 but AFAIK iTunes 7 was the same (ie converting .wma to other formats when adding files to the library). I don't know about iTunes 8, haven't used it.