I'm still learning jquery, but I think this should work for you...
(textbox with an id of text1 and a checkbox of class checkbox)
<script>
$(document).ready(function(){
$('#text1').prop("disabled", true);
$('.checkbox').click(function() {
if ($("#text1").prop("disabled")) {
$("#text1").prop("disabled", false);
} else {
$("#text1").prop("disabled", true);
}
})
})
</script>