function banner() {
};

banner = new banner();
number = 0;

// bannerArray
banner[number++] = "<a href=http://www.corujinha.com.br/index.htm><img src=http://www.corujinha.com.br/files/banner.gif style='border: 1pt solid black'></a>"
// keep adding items here...

increment = Math.floor(Math.random() * number);

document.write(banner[increment]);