<%@Import Namespace="System.IO"%>
<script language="vb" runat="server">

Sub Page_Load()
  Response.Expires = -1
  
  ' scarico un file di nome file.zip
  Dim strPath as String = Server.MapPath("file.zip")

  Dim file as FileInfo = New FileInfo(strPath)
  ' verifica esistenza
  If file.Exists Then
    ' imposta le headers
    Response.Clear()
    Response.AddHeader("Content-Disposition", "attachment; filename=" & file.Name)
    Response.AddHeader("Content-Length", file.Length.ToString())
    Response.ContentType = "application/octet-stream"

    ' leggo dal file e scrivo nello stream di risposta
    Response.WriteFile(strPath)
    Response.End()
  Else
    Response.write("Impossibile scaricare il file.")
  End If
End Sub

</script>