When I had my Jimmy the same thing happened and it was a some sort of AC filter was crushed/not working. It most likely is trapped water some how. Check with some of the guys that have the Chiltons or maintance manual and see if Alti's have a AC water catcher or a filter of some sort.
What you are smelling is the mildew on the condensor (under the dash). It is caused by water being left on the condensor and allowing mildew to grow.
How to fix it: Open all the doors and windows in your car. Put the fan on 3 and recirculate with the vents on. feel around in the footwell for the vent that is sucking air in. Spray a can of Lysol (pick a scent you like) into the vent that is sucking. Spray the whole can in then shut the fan off. Leave the doors and windows open to let the car air out. You may need to wipe some residue off the vents or the dash if you sprayed it in too fast. Don't use the fan or the AC for at least 4 hours. That should kill any mildew you have in there and stop the smell.
How to prevent it: Every time you use the AC. When you are going to stop the car. Turn the AC off and put the fan on high with outside air until it comes through the same temp as outside. If you can remember to do this everytime it will keep the condensor dry and prevent the growth of mold. It only takes a second or two.
There are sprays and cleaners you can buy. I know some of them you just turn the fan on all the way, have your windows rolled down, and then spray the cleaner through the outside vents( the ones by the wipers) it will clean out the mildew. If you want to go the cheap way, I saw on Motor Trend or Auto Week ( one of those shows on Speed Vision) that you can also use Lysol Spray. They should have the cleaners at any auto parts store. Peace