Confirmation dialog during browser back button or cancel button is clicked.

Hi

Can any one suggest me how to show the confirmation dialog during the back button is clicked.

I use the following javascript.

  window.onunload = function () {if (document.getElementById('ctl00_ContentPlaceHolder1_hdnIsDirty').value != '' & DirtytoRedirect == '') {

                        DirtytoRedirect = 'True';

                        return "Leaving this page will reset the wizard";

                    }

                }

But no use.

Thanks in advance.