求助！vb.net 2010 datagridview另存为2007版本的EXCEL文件打开提示文件损坏，2003正常。

Dim SaveFile As New SaveFileDialog
        SaveFile.Filter = "Excel 2003|*.xls|Excel 2007|*.xlsx"
        SaveFile.OverwritePrompt = True            
        SaveFile.RestoreDirectory = True            
        SaveFile.InitialDirectory = "d:\"           
        SaveFile.ShowDialog()                        
        Dim ws As New (SaveFile.OpenFile, System.Text.Encoding.Default)
        Dim str As String = ""
        For i As Integer = 0 To DGV.Columns.Count - 1
            str += DGV.Columns(i).HeaderCell.Value.ToString & vbTab
        Next
        str += vbCrLf
        For Each row As DataGridViewRow In DGV.Rows
            For i As Integer = 0 To DGV.Columns.Count - 1
                If Not row.IsNewRow Then
                    str += row.Cells.Item(i).Value.ToString & vbTab
                End If
            Next
            str += vbCrLf
        Next
        ws.Write(str)
        ws.Close()
    End Sub