When I use my cars AC/Heat/Vent I have a horrible musty smell?

I'd love to be able to fix this on my own, but i somehow doubt it. I have a 2000 ford taurus. The AC, Heat and vent all produce this horrible musty smell that showed up out of nowhere and is quite strong, not disappearing. Is this condensation somewhere in there, mold/mildew? My big curiosity is how much this will run me to get fixed? Anyone have experience with this?