My house is constantly dusty, and I know it comes from the central air conditioning / heating system. But I learned of a way of "filtering" the air as it comes from the vents and keeping your house a bit more dust-free (and allergen-free). You should always purchase a good-quality filter for your system and replace it regularly, as your unit recommends. The dirtier it gets, the more dust it will blow around your house (yuck). Then, use a piece of cheesecloth under the vents to filter the air before it blows into your house. Pull the register out, stretch a piece of cheesecloth over the bottom of it and put it back in its slot (checking it to make sure it covers the whole vent but doesn't stick out). Nifty, huh?