Suddenly, with no changes to my settings, every file that I download from Limewire causes my ITunes to say "Cannot convert to MP3" and I am unable to move any of them into the library. What to do? I am frightened and alone.

What format is the music you download? mp3? Try opening the songs in Quicktime Player, and if that doesn't work, try downloading something very popular with lots of sources. This might be a problem with iTunes so you could also try re-installing it.