private void button1_Click(object sender, EventArgs e)
    {
        SubForm sf = new SubForm();
        sf.FormClosing += new FormClosingEventHandler(Subform_Closing);
 
        button1.Enabled = false;
 
        sf.Show();
    }
 
 
    private void Subform_Closing(object sender, FormClosingEventArgs e)
    {
        button1.Enabled = true;
    }