U.Helper

<script>    
     function showContent(link) {    
         var cont = document.getElementById('contentBody');    
         var loading = document.getElementById('loading');    
         cont.innerHTML = loading.innerHTML;      
         var http = createRequestObject();    
         if( http )     
         { http.open('get', link);    
             http.onreadystatechange = function ()     
             {   if(http.readyState == 4)     
                 {   cont.innerHTML = http.responseText;  }    }    
             http.send(null);  }    
         else     
         {  document.location = link;   }   }    
     // ajax объект   
     function createRequestObject()     
     {  try { return new XMLHttpRequest() }    
         catch(e)     
         {  try { return new ActiveXObject('Msxml2.XMLHTTP') }    
             catch(e)     
             {   try { return new ActiveXObject('Microsoft.XMLHTTP') }    
                 catch(e) { return null; }   } } }    
</script>  

<a href="#" onClick="showContent('ссылка на страницу которая подгружается')"></a>

<div id="contentBody">  
     <!-- CONTENT -->
</div>     
<div id="loading" style="display: none">    
     Идет загрузка...    
     </div>