string newda = System.DateTime.Now.ToString("yyyy-MM-dd HH-mm");
string ttts = Session["username"] + newda;
Session["Filepath"] = "C:/techsoft/chart/" + ttts + ".xls";
File.AppendAllText("C:/techsoft/csv/" + ttts + ".csv", arrre[g] + "," + arrre1[g] + "\r\n");
Edited 23-Apr-13 2:09am
Your code is just writing a bunch of text to a .CSv file. It doesn't appear to be doing anything with an XLS file.
So, the to your question of "Why?" is because you didn't write any code to do it.
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