private void button1_Click(object sender, EventArgs e)
{
    this.timer1.Enabled = !(this.timer1.Enabled);
    this.button1.Text = (this.timer1.Enabled) ? "Stop" : "Start";
}

private void timer1_Tick(object sender, EventArgs e)
{
    this.nodes.MoveAll();
    this.Refresh();
}