[JsLoveR]

TSprinter,

function clearCheckBoxes() {
	var form=document.forms['main_form'];
	for(var i=0;i<form.elements.length;i++) {
		if(form.elements[i].checked) {
			form.elements[i].checked=false;
		}
		
	}
}

<form name="main_form">
<input type="checkbox" checked="checked"/><br/>
<input type="checkbox" checked="checked"/><br/>
<input type="checkbox" checked="checked"/><br/>
<input type="checkbox" checked="checked"/><br/>
</form>
<p onclick="clearCheckBoxes()">кликни</p>