You should add a quick check whether the file exists, and if it does remove it before trying to save the new document.
I'm not entirely sure if this will remove the save as dialog, but it's worth a try.
if( File.Exists("csharp.net-informations.xls") )
File.Delete("csharp.net-informations.xls");
xlWorkBook.SaveAs("csharp.net-informations.xls", Excel.XlFileFormat.xlWorkbookNormal, misValue, misValue, misValue, misValue, Excel.XlSaveAsAccessMode.xlExclusive, misValue, misValue, misValue, misValue, misValue);