This study involves high dose chemotherapy followed by return (infusion) of blood stem cells. Stem cells are undeveloped cells that have the capacity to grow into mature blood cells, which normally circulate in the blood stream. The high dose chemotherapy consists of cyclophosphamide and anti lymphocyte antibody (a protein that depletes cells that cause damage to the body). The purpose of the intense chemotherapy is to destroy the immune system completely. The purpose of the stem cell infusion is to restore the body's blood production, which will be severely impaired by the high dose chemotherapy and anti lymphocyte antibody.