Usando Timers em C# (Windows Forms)

private void timer1_Tick(object sender, EventArgs e)
{
  // acrescenta um "X" na barra de títulos a cada
  // disparo do timer
  Form1.ActiveForm.Text = Form1.ActiveForm.Text + " X ";
}

private void button1_Click(object sender, EventArgs e)
{
  // inicia o timer
  timer1.Start();
}

private void button2_Click(object sender, EventArgs e)
{
  // pára o timer
  timer1.Stop();
}