try 
{ 
    AdHtmlString = "<div><a href=\"javascript:\" onclick=\"window.open ('http://ad.gayaha.net/ImageView.aspx?Url=http://ad.gayaha.net/images/20100902221501461.jpg', 'ADOpen', 'width=939, height=383, left='+ ((window.screen.width-939)/2) +', top='+ ((window.screen.height-383)/2) +', toolbar=no, menubar=no, scrollbars=no, resizable=no, location=no, status=no')\"><img src='http://ad.gayaha.net/images/20100902221445540.gif' border='0' alt='3064'   /></a></div>"; 
    document.write(AdHtmlString); 
} 
catch(e) 
{ 
    document.write('广告'); 
}