注册时倒计时的东西

<form action="reg.asp?action=apply" method=post name=agree></td>

<td align=center class=tablebody2><input type="submit" value="请认真阅读本条款和声明 (5 秒后继续)" name="agreeb" class=bk>
<input onclick=history.back(-1) type="reset" value=" 我 不 同 意 " class=bk></td></form></tr>
</table>
<SCRIPT language=javascript>
<!--
var secs = 5;
document.agree.agreeb.disabled=true;
for(i=1;i<=secs;i++) {
window.setTimeout("update(" + i + ")", i * 1000);
}
function update(num) {
if(num == secs) {
document.agree.agreeb.value ="已经 认真阅读本条款和声明　　　并且我 同 意 ";
document.agree.agreeb.disabled=false;
}
else {
printnr = secs-num;
document.agree.agreeb.value = "请认真阅读本条款和声明 (" + printnr +" 秒后继续)";
}
}
//-->
</SCRIPT>