Frankie of Avalon played an out of tune version of the song.
Behind the scenes
It was used as the main title theme for the early King's Quest games. It first appeared in the 1987 KQ1 ports (except Apple IIGS) and all versions of KQ2 (though in 1985 1.1H version of KQ2 it is used as the theme inside of Castle Daventry and not played on the title screen). It was made part of the title theme in the 1987 rereleases of KQ2. KQ1 for Apple II has a different title theme.
As almost everyone knows, Greensleeves is a well known traditional English folk song.