She is barking because she thnks she rules the pack. In order for you to get her to stop barking, you first have to establish that you are the pack leader.
Here is a link to the dog Whisperer website. He has a lot of good information there on how to establish yourself as the leader of the pack.
http://www.dogchannel.com/experts/cesar/default.aspx
If you take it seriously and follow is suggestions, it really does work.
Also try to catch a few of his shows on The National Geographic Channel. A new series starts tomorrow night,. If you don't have cable perhaps a friend could tape it for you.