download files(csv,zip) from web site and convert it to xls.

Hi, how about something like this:

using (WebClient client = new WebClient())
{
    string url = "http://www.nse-india.com/content/nsccl/oi_cli_limit_07-JAN-2013.lst";
    string downloadedFile = "oi_cli_limit_07-JAN-2013.lst";
    string convertedFile = "oi_cli_limit_07-JAN-2013.xls";
 
    client.DownloadFile(url, downloadedFile);
    ExcelFile.Load(downloadedFile, LoadOptions.CsvDefault)
             .Save(convertedFile, SaveOptions.XlsDefault);
}

The code uses a .NET component for excel spreadsheets and as you can see all you need to do is load an input (from a path or from a stream) and save as an output of the required format (again to a path or to a stream).