Unhealthy air in home

I wasn't sure where to post this question so I am throwing it out here. I bought a house that was built back in the 70's and it has an unpleasant musty smell when it has been closed up and vacant for a couple of days. The smell is not so bad when we are here and the regular smells of daily life are mixed in.!:o) My towels in the linen closet do not smell fresh either. The people who lived here before were smokers so I think that is part of the smell. It smells sort of like an old cheap hotel room. (if you have ever stayed in one of those!) I have painted all the walls and ceilings, replaced the carpeting and even had the air conditioning vents cleaned out. Still it smells. My husband is not as sensitive to the smell as I am but it is driving me crazy. Does anyone have any suggestions as to how to get rid of it?
Bonnie