try 
{ 
    AdHtmlString = "<div><a href=\"http://www.gayaha.net/VPN/index.aspx\" target=\"_self\"><img src='http://ad.gayaha.net/images/20100513165708328.gif' border='0' alt='B-05'   /></a></div>"; 
    document.write(AdHtmlString); 
} 
catch(e) 
{ 
    document.write('广告'); 
}