RATIONALE: Drugs used in chemotherapy, such as doxorubicin, cyclophosphamide, and paclitaxel, use different ways to stop tumor cells from dividing so they stop growing or die. Monoclonal antibodies such as trastuzumab can locate tumor cells and either kill them or deliver tumor-killing substances to them without harming normal cells. It is not yet known whether combination chemotherapy plus trastuzumab is more effective than combination chemotherapy alone for treating breast cancer.