Excel SaveAs not working

I am running an Excel 2007 vba program from dos (using cscript). The program opens an Excel file that has a fileformat of txt. It uses the data in the file to create two charts for each day. So far everything works except when saving the file. I would ultimately like to save it back with the same name, but needing to use a different fileformat so that the charts will display. I have tried several combinations of the saveas command. The weird thing is, when I use the := in the saveas command, the program doesn't even open Excel - but when I use only = in the command it opens the file and creates the charts. Here is the program:

Set objExcel = CreateObject("Excel.Application")
objExcel.Visible = True
Set objWorkbook = objExcel.Workbooks.Open("Z:\Num_Doc_BP_2009_test.xls")


'***************************************************************************
' Create area charts for each day's Number of Documents.                   *
'***************************************************************************

start_value = 2
end_value = 25

range_value = "='" & objWorkbook.ActiveSheet.Name & "'!$C$" & start_value & ":$C$" & end_value & ""
    
Do While objWorkbook.ActiveSheet.Cells(end_value,1).Value <> ""

    x_value = "='" & objWorkbook.ActiveSheet.Name & "'!$B$" & start_value & ":$B$" & end_value & ""
    
    objWorkbook.ActiveSheet.Shapes.AddChart.Select
    objWorkbook.ActiveChart.ChartType = 76
    
    chart_range = "='" & objWorkbook.ActiveSheet.Name & "'!$F$" & start_value & ":$P$" & end_value & ""
        
    Set RngToCover = objWorkbook.ActiveSheet.Range(chart_range)
    Set ChtOb = objWorkbook.ActiveChart.Parent
    
    ChtOb.Height = RngToCover.Height ' resize
    ChtOb.Width = RngToCover.Width ' resize
    ChtOb.Top = RngToCover.Top ' reposition
    ChtOb.Left = RngToCover.Left ' reposition

    objWorkbook.ActiveChart.SeriesCollection.NewSeries
    objWorkbook.ActiveChart.SeriesCollection(3).Name = "='" & objWorkbook.ActiveSheet.Name & "'!$C$1"
    objWorkbook.ActiveChart.SeriesCollection(3).Values = range_value
    objWorkbook.ActiveChart.SeriesCollection(4).Delete
    objWorkbook.ActiveChart.SeriesCollection(2).Delete
    objWorkbook.ActiveChart.SeriesCollection(1).Delete
    objWorkbook.ActiveChart.SeriesCollection(1).XValues = x_value
    
    
    start_value = start_value + 24
    end_value = end_value + 24
    
    range_value = "='" & objWorkbook.ActiveSheet.Name & "'!$C$" & start_value & ":$C$" & end_value & ""
    
    Loop
    
'***************************************************************************
' Create area charts for each day's Number of Business Processes.          *
'***************************************************************************
    
start_value = 2
end_value = 25

range_value = "='" & objWorkbook.ActiveSheet.Name & "'!$D$" & start_value & ":$D$" & end_value & ""
    
Do While objWorkbook.ActiveSheet.Cells(end_value,1).Value <> ""

    x_value = "='" & objWorkbook.ActiveSheet.Name & "'!$B$" & start_value & ":$B$" & end_value & ""
    
    objWorkbook.ActiveSheet.Shapes.AddChart.Select
    objWorkbook.ActiveChart.ChartType = 76
    
    chart_range = "='" & objWorkbook.ActiveSheet.Name & "'!$R$" & start_value & ":$AA$" & end_value & ""
        
    Set RngToCover = objWorkbook.ActiveSheet.Range(chart_range)
    Set ChtOb = objWorkbook.ActiveChart.Parent
    
    ChtOb.Height = RngToCover.Height ' resize
    ChtOb.Width = RngToCover.Width ' resize
    ChtOb.Top = RngToCover.Top ' reposition
    ChtOb.Left = RngToCover.Left ' reposition

    objWorkbook.ActiveChart.SeriesCollection.NewSeries
    objWorkbook.ActiveChart.SeriesCollection(3).Name = "='" & objWorkbook.ActiveSheet.Name & "'!$D$1"
    objWorkbook.ActiveChart.SeriesCollection(3).Values = range_value
    objWorkbook.ActiveChart.SeriesCollection(4).Delete
    objWorkbook.ActiveChart.SeriesCollection(2).Delete
    objWorkbook.ActiveChart.SeriesCollection(1).Delete
    objWorkbook.ActiveChart.SeriesCollection(1).XValues = x_value
    
    
    start_value = start_value + 24
    end_value = end_value + 24
    
    range_value = "='" & objWorkbook.ActiveSheet.Name & "'!$D$" & start_value & ":$D$" & end_value & ""
    
    Loop


objExcel.ActiveWorkbook.SaveAs Filename:="C:\Num_Doc_BP_2009_test.xlsx", FileFormat:=xlOpenXMLWorkbook