Q:

Cannot download a subsite with a video

I've made a simple program (closely resting on this example)  that visualises my problem. It only downloads a file from specified URL, nothing more; everything was OK until I tried with such data:

http://www.youtube.com/watch?v=pqaARDsiJv4

Surprisingly, nothing happens. Why it doesn't download neither the related video nor the HTML source code?
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Net;
using System.IO;
using System.ComponentModel;

namespace downloader
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void DownloadButton_Click(object sender, RoutedEventArgs e)
        {
            WebClient client = new WebClient();

            string fileName;

            try
            {
                fileName = System.IO.Path.GetFileName(URLBox.Text);
                Uri currentURL = new Uri(URLBox.Text);

                client.DownloadFileCompleted += new AsyncCompletedEventHandler(Completed);
                client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(ProgressChanged);

                client.DownloadFileAsync(currentURL, fileName);
            }
            catch (Exception exc)
            {
                MessageBox.Show(exc.Message);
                progressBar.Value = 0;
            }

        }

        private void Completed(object sender, AsyncCompletedEventArgs e)
        {
            MessageBox.Show("Download Completed!");

        }

        private void ProgressChanged(object sender, DownloadProgressChangedEventArgs e)
        {
            progressBar.Value = e.ProgressPercentage;

        }

    }
}

A:

For such an URL this line...
fileName = System.IO.Path.GetFileName(URLBox.Text);

...sets fileName to "watch?v=pqaARDsiJv4", which is not a legal file name.

