private void button1_Click(object sender, EventArgs e)
        {
            string Username = tbUser.Text;
            string Password = tbPassword.Text;
            string path = @"C:/data.txt";
            string NewUser = "New User......";
            if (!System.IO.File.Exists(path))
            {
                System.IO.File.Create("C:/data.txt");
            }

            string line1, line2, line3, line4, line5, line6, line7, line8, line9, line10;
            using (StreamReader reader = new StreamReader(path))
            {
                line1 = reader.ReadLine();
                line2 = reader.ReadLine();
                line3 = reader.ReadLine();
                line4 = reader.ReadLine();
                line5 = reader.ReadLine();
                line6 = reader.ReadLine();
                line7 = reader.ReadLine();
                line8 = reader.ReadLine();
                line9 = reader.ReadLine();
                line10 = reader.ReadLine();
            }
            if (cb1.Text == NewUser && (line1 == ""))
            {
                tbUser.Text = "Username";
                tbPassword.Text = "Password";
            }
            else if (cb1.Text == NewUser && (line4 == ""))
            {
                tbUser.Text = "Username";
                tbPassword.Text = "Password";
            }
            else if (cb1.Text == NewUser && (line6 == ""))
            {
                tbUser.Text = "Username";
                tbPassword.Text = "Password";
            }
            else if (cb1.Text == NewUser && (line8 == ""))
            {
                tbUser.Text = "Username";
                tbPassword.Text = "Password";
            }
            else if (cb1.Text == NewUser && (line10 == ""))
            {
                tbUser.Text = "Username";
                tbPassword.Text = "Password";
            }