Javascript - Make Code Works If Used Inside Update Panel And Checkbox Values Will Be Disabled According To Database Field?

This code works fine if i use this inside ssercontrol > panel and i have a checkboxes in table when no checkbox is checked its works fine .... but if i disabled and checked any of the textbox then this doesn't work .... in usercontrol why ? i didnt understand ..

<script type="text/javascript" language="javascript">
function checkboxChecked(){
var allInputs = document.getElementsByTagName("input");
for(var i=0; i<allInputs.length; i++) {
var chk = allInputs[i];
if(chk.type == "checkbox" && !chk.disabled && chk.checked) {

return true;
}
}
alert("OOps! You haven't selected all available checkboxes");
return false;
}
</script>