FOR THE MACARONI: Bring to a boil enough water to cover amply the macaroni. Add salt. Boil rapidly for 10 minutes. Reduce heat and boil slowly for 20 minutes more, adding more boiling water if necessary. Drain. Rinse with cold water. Chill. Mix with following dressing.
DRESSING: Whip the cream stiff. Add salt to taste, sugar and vinegar, a few drops at a time. Mix well and add the horseradish. Fold into the chilled macaroni. Serves 6.