Problem with air conditioning

Hi,
 
The air conditioning in my Primera P12 is driving me crazy. I am only using it once or twice a week, and every time I have used it and turn it off, the air starts to smell pretty bad. It is hard to describe the smell, but 'dry', 'dusty', 'carbon-like' are the words that come to mind.
 
While AC is turned ON, there are no problems at all. It works great, and there is no smell.
 
I have tried treating it with a product called "Bell Add Aircondition Cleaner", which may - or may not - have helped a bit. But not significantly.
 
I am starting to think about disassembling the air condition system to make sure it is clean, but I would rather not - and I am not sure how to do it (if it is accesible at all).
 
Any advice would be great :-)

For smell you can buy an AC Fogger .......Not sure what the Bell Add thing was... but really my only other thought is clean your motor with orange clean by hand ..... you will def. get an orange smell in the car.

What is an AC Fogger?
 
The Bell Add thing is a disinfective spray that is sprayed into the air intake with the fan running.