Place potatoes into a large saucepan, and fill with enough water to cover. Bring to a boil, and cook for about 8 minutes, or until tender.
Drain, and cool slightly.
In a large bowl, stir together the mayonnaise and brown sugar. Stir in the sweet potatoes, walnuts and raisins until evenly coated. Chill for at least 1 hour before serving.