If you can live with the buttons being labelled "OK" and "Cancel", then you can use
the confirm
method[
^].
if(confirm("...")){
location.href = "OkPage.aspx";
}
else {
location.href = "CancelPage.aspx";
}
If you need custom text on the buttons, then you'll need a custom Javascript dialog.
jQuery UI[
^] is a good place to start.