просто зарегистрируйтесь для FormClosing
события Form
void MyClosingEvent(object o, FormClosingEventArgs args)
{
}
private void aboutToolStripMenuItem_Click(object sender, EventArgs e)
{
Form1 form1 = new Form1();
form1.FormClosing += new FormClosingEventHandler(MyClosingEvent);
//Or if you have C# 2 or higher:
//form1.FormClosing += MyClosingEvent;