C#

void CheckBox1_CheckedChanged()  // or something like that
{
    txtBox1.Enabled = CheckBox1.Checked;
}

public class Form1 : System.Windows.Forms.Form
{
  private Thread trdTcpActivityListener = null;

  public Form1()
  {
     InitializeComponent();

     // Initialize seperate thread and start it
     trdTcpActivityListener = new Thread(new ThreadStart(this.Run));
     trdTcpActivityListener.Start();      
      
     // Instead of - this.Run();
  }

  public void Run()
  {
    TcpListener tcpListener = new TcpListener(IPAddress.Any, 65000);
    tcpListener.Start();

    while (true)
    {
      // Use .Pending() to determine if there is any connections
      if (tcpListener.Pending())
      {
	Socket socketForClient = tcpListener.AcceptSocket();
        socketForClient.Close();
	// Maximize the form
	this.WindowState = FormWindowState.Maximized;      
      }

      // Make sure the while loop doesn't hog the CPU
      Thread.Sleep(30);
    }
  }
}