Secure File Type With Password On Creation?

I am currently writing a gridview to an excel file, but i cant make it password protected (or the password is too easy to break).

Is there another file type that i can write the gridview data to which can be securely password protected at the same time?

I am going to be emailing this file, and i want the end user to have to type in a password to open it.

my current code:

Response.Clear();
Response.AddHeader("content-disposition", "attachment;filename=FileName.xls");
Response.Charset = "";
Response.ContentType = "donedeals/vnd.xls";
System.IO.StringWriter stringWrite = new System.IO.StringWriter();
System.Web.UI.HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWrite);
GridView1.RenderControl(htmlWrite);
Response.Write(stringWrite.ToString());
Response.End();