Didn't intend to make light of the importance of unrestricted dryer venting, it's a safety deal to keep that vent clean. (I've never heard of a vent running vertical to the roof but what it is is what it is - it needs to be cleaned. I take my flex line off every year and blast it out with the garden hose into the neighbors yard - kidding - but I do clean it every year as there is always a bunch of lint plus the dryer works about twice as fast after cleaning the hose).
Hey, if you are handy whatsoever, Home Depot or Lowes sells a device like
http://www.rewci.com/lirodrveclki.html where you can clean the line yourself.
Now airconditioning ducts are a different deal - some contractors say how much that is needed - I disagree - as the return vent air goes straight through a furnace filter to catch dust. The supply duct is downstream of the filter so whatever was there after construction would have certainly purged itself within the three years you have lived in this house. (This Munn guy mentioned by Allair.... sounds like an honest guy that I would trust and no I am not Munn or even related...I just like honest people not willing to cheat people)
So clean the dryer vent, forget about the HVAC ducts, make smiley faces on tables until you replace the carpet with a better grade or go to tile or hardwood floors.
Great question Moongirl!