Enable and disable button using javascript and asp.net

I am checking if user exists in database if exists I am showing message as "existed user" and then I need to disable the signup button if not I need to enable it.

I am unable to enable and disable the sign Up button.

Can anyone help me in this issue?

Here is my code:

 <script type="text/javascript">
     $(function () {
         $("#<% =btnavailable.ClientID %>").click(function () {
             if ($("#<% =txtUserName.ClientID %>").val() == "") {
                 $("#<% =txtUserName.ClientID %>").removeClass().addClass('notavailablecss').text('Required field cannot be blank').fadeIn("slow");

             } else {
                 $("#<% =txtUserName.ClientID %>").removeClass().addClass('messagebox').text('Checking...').fadeIn("slow");
                 $.post("LoginHandler.ashx", { uname: $("#<% =txtUserName.ClientID %>").val() }, function (result) {
                     if (result == "1") {
                         $("#<% =txtUserName.ClientID %>").addClass('notavailablecss').fadeTo(900, 1);
                       document.getElementById(#<% =btnSignUp.ClientID %>').enabled = false;
                     }
                     else if (result == "0") {
                         $("#<% =txtUserName.ClientID %>").addClass('availablecss').fadeTo(900, 1);
                        document.getElementById('#<% =btnSignUp.ClientID %>').enabled = true;
                     }
                     else {
                         $("#<% =txtUserName.ClientID %>").addClass('notavailablecss').fadeTo(900, 1);
                     }
                 });
             }
         });

         $("#<% =btnavailable.ClientID %>").ajaxError(function (event, request, settings, error) {
             alert("Error requesting page " + settings.url + " Error:" + error);
         });
     });
</script>