remove the event handler (by click the lightning bolt at the top of the properties pane. then clear the SelectedIndexChanged handler).
and in constructor code, first set the SelectedValue, and then add the event handler. here:
public Form1()
{
InitializeComponent();
comboBox1.SelectedIndex = 0;
comboBox1.SelectedIndexChanged += combobox_SelectedIndexChanged;
}
private void combobox_SelectedIndexChanged(object sender, EventArgs e)
{
Process.Start(Application.ExecutablePath);
this.Close();
}