Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED))

Now I am trying to use web service to send out a fax by dll.
In dll, the function is as following:
 public void SendFax(string DocumentName, string FileName, string RecipientName, string FaxNumber)

        {

            if (FaxNumber != "")

            {

               
StreamWriter myStream = new
StreamWriter(@"c:\windows\temp\yyyzzz1.txt");

                myStream.WriteLine("asdfghjkl");

                try

                {

  &nb ...