RATIONALE: Giving chemotherapy drugs and total-body irradiation before a donor stem cell helps stop the growth of cancer or abnormal cells. It may also stop the patient's immune system from rejecting the donor's stem cells. When the healthy stem cells from a donor are infused into the patient they may help the patient's bone marrow make stem cells, red blood cells, white blood cells, and platelets. It is not yet known which combination chemotherapy regimen is most effective when given before a donor stem cell transplant in treating aplastic anemia or hematologic cancer.