[dionis]

<p>Welcome to the site <b id='boldStuff2'>****</b> </p>
       <script type="text/javascript">
            var url = "http://google.com";
            var a = '<a href="' + url + '">' + url + '</a>';
            document.getElementById('boldStuff2').innerHTML = a;            
        </script>

либо

<p>Welcome to the site<b><a id="a1"></a></b> </p>
        <script type="text/javascript">
            var url = "http://google.com";
            document.getElementById('a1').innerHTML = url;
            document.getElementById('a1').href = url;
        </script>