Do you mean a JS message box or an area of the existing HTML?? I have solutions for both without affecting any existing HTML...
>>Usually hidden frame is used for this. Load page in that hidden frame when main frame remains unchanged. The HTML page loaded to the hidden frame contains a script that do anything you want to refresh on the pages without reloading them, include showing of the message as you require. (I usually use this approach to refresh 2 related list - when item in one list changed, another list refreshed by loading list from server. For example, States list - list of cities refreshing, County selection - refresh of list of zip codes in that county, order selection - refresh list of items etc.) Works in IE, Netscape and Opera.
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>Yes, that'll work. However, I don't want to add a frame in all HTML pages which do not include frame. That represents several dozens of pages and they are within the main browser page.