Add an Infowindow to a CartoDB layer (using createLayer).

<!DOCTYPE html>
<html>
  <head>
    <title>Leaflet multilayer example | CartoDB.js</title>
    <meta name="viewport" content="initial-scale=1.0, user-scalable=no" />
    <meta http-equiv="content-type" content="text/html; charset=UTF-8"/>
    <link rel="shortcut icon" href="https://cartodb.com/assets/favicon.ico" />
    <style>
      html, body, #map {
        height: 100%;
        padding: 0;
        margin: 0;
      }
    </style>

</head>
  <body>
    <div id="map"></div>

<script src="https://libs.cartocdn.com/cartodb.js/v3/cartodb.js"></script>
    <script>

function main() {

// create leaflet map
        var map = L.map('map', { 
          zoomControl: false,
          center: [43, 0],
          zoom: 3
        })

// add a base layer
        L.tileLayer('https://tile.stamen.com/toner/{z}/{x}/{y}.png', {
          attribution: 'Stamen'
        }).addTo(map);

// add cartodb layer with one sublayer
        cartodb.createLayer(map, {
          user_name: 'examples',
          type: 'cartodb',
          sublayers: [{
             sql: 'select * from country_boundaries',
             cartocss: '#layer { polygon-fill: #F00; polygon-opacity: 0.3; line-color: #F00; }',
             interactivity: 'cartodb_id'
          }]
        })
        .addTo(map)
        .done(function(layer) {
          cdb.vis.Vis.addInfowindow(map, layer.getSubLayer(0), ['cartodb_id'])
        });

}

// you could use $(window).load(main);
      window.onload = main;

</script>

</body>
</html>

Modified http://libs.cartocdn.com/cartodb.js/v3/cartodb.js to a secure url

https://libs.cartocdn.com/cartodb.js/v3/cartodb.js