private void treeView1_MouseDown(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Right)
    {
        var myForm = new Form {Text = "My Form"};
        myForm.SetBounds(10, 10, 200, 200);

        myForm.Show();
        // Determine if the form is modal.
        if (myForm.Modal == false)
        {
            // Change borderstyle and make it not a top level window.
            myForm.FormBorderStyle = FormBorderStyle.FixedToolWindow;
            myForm.TopLevel = false;
        }
    }
}