Air Conditionor problems, 2001 Alero

Hello-- I have a 2001 Olds Alero, 4 cylinder, 44,000 miles. The AC only works when turned to the 4th level or higher. The first 3 settings produce nothing or, if on the highway, only the faintest breath of air. But if I turn it up to the 4th level, or higher, the AC is great, it is powerful and cold. Do you know what might be the problem? I have never had the AC serviced, the other than this the car runs well. It wasn't used very often for about 3 years (once a week or less) and was in Boston winters for those same 3 years.

Thanks,