void BtnShowForm2Click(object sender, System.EventArgs e)
		{
			Form2 f = new Form2();
			f.Closing += new System.ComponentModel.CancelEventHandler( _
  this.Form2_Closing);
			(sender as Button).Enabled = false;
			f.Show();
			// oder einfach
			// f.ShowDialog();
		}
 
		void Form2_Closing(object sender, EventArgs e){
			this.btnShowForm2.Enabled = true;
		}