var banners = new Array(); 
banners[0] = '<script language="javascript" src="http://adserving.unibet.com/ad.aspx?pid=30284&bid=12049"></script>';
//banners[1] = '<script language=javascript src="http://adserving.unibet.com/ad.aspx?pid=19482&amp;bid=6514"></script>';
index = Math.floor(Math.random() * banners.length);
document.write(banners[index]);