Downloading files using Webclient [RESOLVED]

I'm attempting to use a Webclient to download files from the internet, but I am running into problems. So far I have created a simple test project to get it working correctly, the code is below:

VB Code:

Private Sub cmdDownload_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdDownload.Click
        Const PACKET_SIZE As Integer = 1024 ^ 2
        Dim ch As Char()
        ReDim ch(PACKET_SIZE)
 
        'Create the webclient and open a download stream
        Dim WC As New Net.WebClient
        Dim sr As New IO.StreamReader(WC.OpenRead(txtSrc.Text))
        Dim sw As New IO.StreamWriter(txtDest.Text, False, System.Text.Encoding.Default)
        Dim NumRead As Integer
 
        'Download the file
        lblProg.Text = "Downloading File..." : Application.DoEvents()
        While sr.Peek >= 0
            NumRead = sr.ReadBlock(ch, 0, PACKET_SIZE)
            sw.Write(ch, 0, NumRead)
        End While
 
        'Tidy up
        sw.Close() : sw = Nothing
        sr.Close() : sr = Nothing
        WC.Dispose() : WC = Nothing
 
        lblProg.Text = "Operation Complete"
    End Sub

However, the problem that I'm having is that the webclient doesn't always download a complete file. It will download text files without a hitch, but when you try and use it on something like a zip or executable it will download only a fraction of the file - usually about two thirds.

I have a suspicion the problem lies with the StreamWriter - could it be that the streamwriter is not writing the non-printable characters, hence why it works on text files correctly?