$(document).ready(function () {

      $('#ctl00_ContentPlaceHolder1_AcceptTerms').click(function () {

        var state = $('#ctl00_ContentPlaceHolder1_AcceptTerms').is(':checked');

        //alert( $('#ctl00_ContentPlaceHolder1_Button1').attr("disabled") );

        $('#ctl00_ContentPlaceHolder1_Button1').attr("disabled", state==false);


    });

});