problem with SaveAs method of excel in IIS 5
 

I want to transform excel to html.
I create a object with
Set excelObj = Server.CreateObject("Excel.Application")

and use the object as following
excelObj.DisplayAlerts = False
excelObj.Interactive = False
excelObj.ScreenUpdating = False
excelObj.Workbooks.Open TempPath & repFileName & ".xls"
excelObj.Workbooks(1).Activate
excelObj.Workbooks(1).RefreshAll
excelObj.Workbooks(1).SaveAs TempPath & repFileName & ".htm",44

An error is issued at the last line:
Microsoft Excel error '800a03ec'
Method SaveAs of class Workbook is invalid.

what's wrong? permissions?