take a html Check Box
<input type="checkbox" id="chk1" name="chk" />
<asp:LinkButton ID="lnk_Register" OnClientClick="javascript:return validate();" runat="server" </asp:LinkButton>
<pre lang="xml"><script type="text/javascript">
function validate() {
if (document.getElementById('chk1').checked == true) {
return true;
}
else {
alert("You Must Agree To Terms");
return false;
}
}
</script>