ThreadStart start = delegate()
{
    try
    {
        string tmp = Path.Combine(path, "tmp");
        if (!Directory.Exists(tmp)) Directory.CreateDirectory(tmp);
        ZipHelper.Decompress(Path.Combine(path, "data.zip"), tmp);
        result = string.Empty;
    }
    catch (Exception ex)
    {
        result = ex.Message;
    }
};

// Create the thread and kick it started!
this.Dispatcher.Invoke(DispatcherPriority.Background,
(Action)delegate()
{
    Thread thread = new Thread(start);
    thread.Start();
    thread.Join();
});