"Deleted" info keeps coming back

I have been plowing throught the topics and couldn't find an answer:

Among all the information about an mp3 file I want to keep there's also often, as we all know, the "composer" (of the particular song) information. Try to delete it thouroug so many ways, now also with the Mp3tag-programme, but it keeps coming back on the information tab when I rightclick the selectet file, and by "selected" I mean the file played by my media player - this media player (just the usual Windows M.P.) keeps showing this unwanted "composer" info!!

Does anybody have an idea how to get rid of this for good?

Mixed up artist and titles in Windows Media Player

... just like mike_nl wrote or you might have APE tags in your files ...

Thanks guys, sounds pretty logical. After re-tagging the collection I'll renew the library.