var ievs = (/MSIE (\d+\.\d+);/.test(navigator.userAgent));

// IF BROWSER IS IE 6 LOAD IE6 STYLE
if (ievs){
   var iev=new Number(RegExp.$1);
   if (iev==6) {
       document.write('<h3>Stai usando il browser: explorer 6, aggiornati al 7!</h3>');
   }
}
// ID BROWSER IS IE 7 LOAD IE7 STYLE
if (ievs){
   var iev=new Number(RegExp.$1);
   if (iev==7) {
       document.write('<h3>Stai usando il browser: Explorer 7</h3>');
   }
}