try 
{ 
    AdHtmlString = "<div><a href=\"http://book.gayaha.com/\" target=\"_blank\"><img src='http://ad.gayaha.net/images/20100329143620896.gif' border='0' alt='IN2'   /></a></div>"; 
    document.write(AdHtmlString); 
} 
catch(e) 
{ 
    document.write('广告'); 
}