if (typeof(window.event)!='undefined')//ie
   {document.onkeydown=function()
                       {var tag_name=event.srcElement.tagName.toUpperCase();
                        if (tag_name!='INPUT' && tag_name!='TEXTAREA')
                           {return event.keyCode!=8;
                           }
                       };
   }
else //firefox, chrome
    {window.addEventListener('keydown',function(evt){if (evt.keyCode==8){evt.preventDefault();}}, true);
    }