Like Tony said. Check the water drain tube at the firewall down under the heater water valve under the hood. With the fan on high, you should be able to feel air coming out of the rubber tube. Its purpose is to allow the water that condenses on the evaporator to drain away.
They sell stuff that works really good at cleaning the evaporator and heater core, and the inside case. BUT...it cost like $25 for a kit.
Try this first.
With the car running, a/c on, I just put the controls to high fan, vents, and outside air (not recirc). Then I go out and spray 1/2 a can of Lysol (with lemon scent) into the air intake grill in front of windshield on the passenger side.
I then go inside and switch vents to floor for a minute, then defrost. That way the Lysol will get everywhere.
Do it again when it smells bad. I do mine every summer. :biglaugh: