IE 7/8 javascript Return false issue

I ve been working on js for last 2 days, but when i use return false in client side validation, the page went postback. Bt actually i want to prevent post back. Only IE 7/8 shows this problem. This issue was solved by
"event.returnValue = false;"

CodeProject
 if (confirm(confirmMessage) == false) {
                event.returnValue = false;
                return false;
            }
            else {
                return true;
            }