-- EDIT --
It sure looks like your code is forcing a radio input to behave like a checkbox input. You might think about just using a checkbox input without the need for jQuery. However, if you want to force, like @manji said, you need to store the value outside the click handler and then set it on each click to the opposite of what it is at that time. @manji's answer is correct, I would just add that you should cache jQuery objects instead of re-querying the DOM:
var $radio = $("#radioinstant"),
isChecked = $radio.attr("checked");
$radio.click(function() {
isChecked = !isChecked;
$(this).attr("checked", isChecked);
});