Prova cosi usando onload:
codice:
window.onload=function(){
if(screen.width > 1024)
{
addScript ('http://syndication.exoclick.com/ads.php?type=728x90&login=peppergossip&cat=472&search=&ad_title_color=0000cc&bgcolor=FFFFFF&border=0&border_color=000000&font=&block_keywords=&ad_text_color=000000&ad_durl_color=008000&adult=1&sub=&text_only=0&show_thumb=&idzone=674089&idsite=233439');
}
else
{
addScript('http://syndication.exoclick.com/ads.php?type=300x50&login=peppergossip&cat=472&search=&ad_title_color=0000cc&bgcolor=FFFFFF&border=0&border_color=000000&font=&block_keywords=&ad_text_color=000000&ad_durl_color=008000&adult=1&sub=&text_only=0&show_thumb=&idzone=675613&idsite=233439');
}
}