window.onload = prepareLinks;
function prepareLinks() {
    if(!document.getElementsByTagName) return false;
    var links = document.getElementsByTagName("a");
    for (var i=0; i<links.length; i++) {
        if(links[i].getAttribute("className") == "popup" || links[i].getAttribute("class") == "popup") {
            links[i].onclick = function() {
                popUp(this.getAttribute("href"));
                return false;
            }
        }
    }
}

function popUp(winURL) {
    window.open(winURL,"popup","width=650,height=600");
}