fso虽然可以创建xls文件，但是不能当做Automation来用，你需要创建Excel.Application：
Dim objExcelApp
Set objExcelApp = CreateObject("Excel.Application")
objExcelApp.Visible = False
objExcelApp.Workbooks.Open D:\ExcelExample.xls"
objExcelApp.Cells(4, 3).Value = 100
objExcelApp.ActiveWorkbook.Save
objExcelApp.Workbooks.Close
objExcelApp.Quit
Set objExcelApp = Nothing