Needs["NETLink`"];

xls = NETNew["Microsoft.Office.Interop.Excel.ApplicationClass"];
xls@Visible = True;

wb = xls@Workbooks@Add[]
ws = wb@Worksheets@Item[1]

ws@Cells[1, 1]@Value = 100;
ws@Cells[2, 2]@Value = 200;
ws@Cells[3, 3]@Value = 300;

For[i=1,i<10,i++,
   ws@Cells[i, i]@Value = i*100
]