Sorry this wasn't a more timely response, but I havn't visited here before. There are a few things I feel should be pointed out. Before starting work of this kind, check with your local building dept. to find out if a permit is required and if they allow DIY electrical work. Having the work permited and inspected will help protect you if there is a fire later i.e. don't give the insurance co. any loopholes. Next, installing a fan on an existing box was mentioned; this is not allowed by code unless the box is listed for supporting a fan (lighting fixture boxes are not). Whether installing a new box or replacing an existing one, be sure to get one listed for the purpose, they are readily available. Finally, when running cable in an attic, there are NEC requirements (detailed in article 336) for support, protection, and termination that must be followed. They are not particularly complicated, but must be understood. A excellent book for an electrical DIY, is Electrical Wireing RESDENTIAL by Ray C. Mullin (Delmar Press, www.delmar.com).