public MainForm() {
    Thread thr = new Thread(new ThreadStart(Thread_Method));
}

void Thread_Method() {
    this.listBox1.Invoke(new updateListBoxCallBack(updateListBox));
}

public delegate void updateListBoxCallback();
public updateListBox() {
    // update it in here
}

// since you're checking if it already exists, let's store the contents of the listbox in an array.
String [] current; // contains the current contents of your listbox
String [] temp; // stores the NOW contents of what's going on
// (use to determine whether it's different to the current contents)
// it's up to you to populate it, just loop through your listbox. (I'll show you where)

public MainForm() {
    Thread thr = new Thread(new ThreadStart(Thread_Method));
    // populate current with the contents of your listbox, after it's been filled for the first time
}

void Thread_Method() {
    // run a check to get all the NOW events
    // pseudocode:
    if (current != temp) // if it's changed, then we should update it
        this.listBox1.Invoke(new updateListBoxCallBack(updateListBox));
}

public delegate void updateListBoxCallback();
public updateListBox() {
    // update it in here
}