c#AJAX 用户已注册

js:
<script>
var xmlHttp=false;
try {
xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
} catch (_e) {

try {
xmlHttp = new ActivrXobject("Microsoft.XMLHTTP");
} catch (_E) { }
}

if (!xmlHttp && typeof XMLHttpRequest != 'undefined') {
try {
xmlHttp = new XMLHttpRequest();
}
catch (e) {
xmlHttp = false;
}
}
function checktextbox1() {
var TextBox1 = document.getElementById("TextBox1").value;
if (TextBox1 == "") {
t1.innerHTML = "不能为空";
}
else if (TextBox1 != "