Function takes long time -- Thread was being aborted exception

 Hi all,
I am writing code which has a function that takes too much time for executing.
Function contains:
Dim oExcel As Object = CreateObject("Excel.Application")
        oExcel.Caption = System.Guid.NewGuid.ToString.ToUpper
        ExcelCaption2003 = oExcel.Caption
        Dim oBook As Object = oExcel.Workbooks.Open(RootDir & "\" & sheetname & ".xls")
        Dim oSheet As Object = oBook.Worksheets(1)
        If oBook.SaveAs(RootDir & "\" & sheetname & ".xlsx", XlFileFormat.xlOpenXMLWorkbook) Then
            If Not oExcel.Workbooks Is Nothing Then
              ...