Help cleaning a musty room!?

I live in a two story apartment, and my bedroom is upstairs in what is basically the attic of the building. The room has tiny windows that don't open so there is not a lot of air circulation, but there is a fan. I'm not sure if the musty smell is from the carpet, clothes, or something else. Any general tips to help me get rid of the smell rather than just covering it up (Febreze)?