function init(){
    var tb = document.getElementById("tbl");
    var tds=tb.getElementsByTagName("td");
    for(var i=0;i<tds.length;i++){
        var as=tds[i].getElementsByTagName("a");
        for(var j=0;j<as.length;j++){
            //if(as[j].tagName == "A"){
                as[j].setAttribute("href","http://www.baidu.com");
                alert(as[j].href);
            //}
        }
    }
}

------解决方案--------------------
JScript code

window.onload = doit;

function doit() {
    if(!document.getElementById("tb1")) {
        alert("error");
        return false;
    }
    var table = document.getElementById("tb1");
    var a = table.getElementsByTagName("a");
    for(var i = 0; i < a.length; i++){
        a[i].setAttribute("href","http://forum.csdn.net/SList/JavaScript/");
    }
}