Pierce potatoes several times with a fork and place in a microwave-safe dish. Microwave on high setting for 10 to 12 minutes, until tender and cooked through. Set aside to cool. In a medium bowl, whisk together mayonnaise, vinegar, mustard, horseradish, sugar and salt. Mix in green onions, celery and red onion. Slice potatoes into one-inch cubes; carefully toss with mayonnaise mixture. Refrigerate until ready to serve.