FileSystemWatcher and GUI

I have a little problem with my WPF project and the FileSystemWatcher class. In my MainWindow class the watcher begins to watch a folder when Button Start is clicked in the UI. Everything works without any problems - the watcher recognizes correctly when a file is created. But while watcher is waiting it is not possible for user to do anything in the UI. It should be possible for nexample to click Stop...

 private void Start_Click(object sender, RoutedEventArgs e)
    {
        rdbTextBox.Document.Blocks.Clear();
        Start.IsEnabled = false;
        rdbTextBox.Document.Blocks.Add(new Paragraph(new Run("Test gestarte-Warte auf Befund....")));
        Stop.IsEnabled = true;

        watcher = new FileSystemWatcher(ConfigSettings.Default.FilePath); 

        // Only watch text files.
        // watcher.Filter = "*.bef";
        watcher.Filter = "*.txt";
        // Add event handlers.          
        watcher.Created += OnCreated;
        // Begin watching.
        watcher.EnableRaisingEvents = true;

        // Wait until new file in folder
        watcher.WaitForChanged(System.IO.WatcherChangeTypes.Created);           
        watcher.Dispose();

        // Parse letter
        edifactLetter = parser.ParseDocument(ConfigSettings.Default.FilePath + "\\" + fileName);
        // Validate Letter
        edifactVal.Validate(edifactLetter);

        writeResults();
        Start.IsEnabled = true;           
    }

    private void OnCreated(object sender, FileSystemEventArgs e)
    {   
        FileInfo file = new FileInfo(e.FullPath);
        fileName = file.Name;
    }

Can anyone explain me what I am doing wrong? Thanks!