In a quart size saucepan combine the sugar and cornstarch. Measure the beet liquid. You need 3/4-cup, add water if necessary to bring the amount of liquid to this level. Gradually stir the liquid into the saucepan, stirring to dissolve the cornstarch. Add the vinegar. Bring the mixture to a boil over medium heat, stirring often. When the mixture boils, add the drained beets and the margarine. Simmer for about 5 minutes to tone down the vinegar, and blend the flavors. Serve hot.