Tela de login.

frmLogin.cs

using System;
using System.Windows.Forms;

namespace ExemploLogin
{
    public partial class frmLogin : Form
    {
        private bool _loginOk = false;

        public frmLogin()
        {
            InitializeComponent();
        }        

        private void frmLogin_FormClosing(object sender, FormClosingEventArgs e)
        {
            if (_loginOk)
            {
                DialogResult = DialogResult.OK;
            }
        }        

        private void _btnLogin_Click(object sender, EventArgs e)
        {
            _loginOk = Login(_txtUser.Text, _txtPass.Text);

            if (_loginOk)
            {
                Close();
            }
            else
            {
                MessageBox.Show("Usuario ou senha não confere.");
            }
        }

        bool Login(string user, string pass)
        {
            return _txtUser.Text.Equals("admin") && _txtPass.Text.Equals("123");
        }
    }
}

Program.cs

using System;
using System.Windows.Forms;

namespace ExemploLogin
{
    static class Program
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            frmLogin login = new frmLogin();

            if (login.ShowDialog() == DialogResult.OK)
            {                
                Application.Run(new MainFormEx());
            }
            else
            {
                Application.Exit();
            }
        }
    }
}