How to disable checkboxes in asp.net

Hi All,
I have a problem with check boxes  in asp.net. I have a javascript function that disables the remaining check boxes if one among them is checked. It runs perfectly in basic html and when I use it in aspx page it is creating problems. I think I found the cause of the problem, but no idea how to solve it.  
 function ckGrp(ck){

var ck_arr=document.getElementsByName(ck.name);
var ca_ln=ck_arr.length;
if(ck.checked){
for(var i=0;i<ca_ln;i++){
if(ck_arr[i].id!=ck.id)ck_arr[i].disabled=true;
}
}else{
for(var i=0;i<ca_ln;i++){
ck_arr[i].disabled=false;
}
}
}  <span>
<asp:checkbox id="txtChkBox3" class="element checkbox" name="Group" runat="server" onclick="ckGrp(this);"></asp:checkbox>
<p class="choice"> ...