Those leaky old windows are bound to make your guests uncomfortable or cause the candles to sputter. New weatherstripping—the best solution—requires removing both sash. For an easier fix that you can do with the window in place, Tom Silva suggests adding an extra sash lock.
This treatment is best on sash you can wiggle in and out when the lock is engaged. Remove both lock pieces, and plug the old screw holes with a sliver of wood and wood glue. Link the lock halves together and reinstall them on the sash about a third of the way in from the side of the window. (Don't forget to drill pilot holes for each of the screws first.) Now do the same with a matching sash lock about a third of the way in from the opposite edge. When both locks are closed, the sash should pull tightly together and keep the cold air outside, where it belongs.