var Advert = new Array();
var count = 0;

var userid = 3193179;
var page = 1;
var rndnum = Math.round(Math.random() * 10000);

Advert[count++] = "<a href='http://www.harbor.ru/ads/adclick.php?n=a51250a1' target='_blank'><img src='http://www.harbor.ru/ads/adview.php?what=zone:4&amp;n=a51250a1' border='0' alt=''></a>";

var RNDNum = 0;
for(;;) {
    RNDNum = Math.floor(Math.random()*count);
    if (RNDNum < Advert.length) break;
}
document.write(Advert[RNDNum]);