protected void Button3_Click(object sender, EventArgs e)
    {
        string fileName = "aaa.txt";//客户端保存的文件名
        string filePath = Server.MapPath("DownLoad/aaa.txt");//路径
        System.IO.FileInfo fileInfo = new System.IO.FileInfo(filePath);
        if (fileInfo.Exists == true)
        {
            const long ChunkSize = 102400;//100K 每次读取文件，只读取100Ｋ，这样可以缓解服务器的压力
            byte[] buffer = new byte[ChunkSize];
            Response.Clear();
            System.IO.FileStream iStream = System.IO.File.OpenRead(filePath);
            long dataLengthToRead = iStream.Length;//获取下载的文件总大小
            Response.ContentType = "application/octet-stream";
 switch (Request.Browser.Browser)
  {
  case "Firefox": Response.AddHeader("Content-Disposition", "attachment;filename=\"" + fileName + "\""); break;
  case "AppleMAC-Safari": Response.AddHeader("Content-Disposition", "attachment;filename=\"" + fileName + "\""); break;
  case "IE": Response.AddHeader("Content-Disposition", "attachment;filename=" + ReplaceEtension(ToHexString(fileName))); break;
  default: Response.AddHeader("Content-Disposition", "attachment;filename=" + ReplaceEtension(ToHexString(fileName))); break;
  }
            while (dataLengthToRead > 0 && Response.IsClientConnected)
            {
                int lengthRead = iStream.Read(buffer, 0, Convert.ToInt32(ChunkSize));//读取的大小
                Response.OutputStream.Write(buffer, 0, lengthRead);
                Response.Flush();
                dataLengthToRead = dataLengthToRead - lengthRead;
            }
            Response.Close();
        }
    }

------解决方案--------------------
明白你说的意思了，
弹出的对话框 的三个按钮 打开，保存，取消
你的说直接保存是OK的，如果点打开（如果是DOC），就会用WORD直接打开，存储的是临时文件，这时WORD程序的标题栏，是编码后的特殊字符，对么？