Decrease dust in an old house, forced air
i would suggest having an HVAC contractor visit to examine all of your ductwork.  My guess would be there are wholes, gaps, or rips in the ducts whereupon the system in operation is pulling the dust in from wall cavities, basement or attic (depending on where the ductwork runs).  Once the contractor (or yourself) seals up and/or replaces any ductwork then you could consider having them vacuumed out.  Also, you should probably put this high on your priority list because interior air quality is really important for controlling allergies and general health.  Good luck!