Mildew Smell

My 97 Bravada is getting some miles (just rolled over 200,000) but i've only had it for about two years. Anyways... when I turn on the air conditioner this mildew smell usually comes out of the vents and hits me in the face. Not very pleasant. I've tried covering it up with air fresheners but I need a real solution. If you have any suggestions I would appreciate it. Thanks.