Did you try DwmGetWindowAttribute() function with DWMWA_EXTENDED_FRAME_BOUNDS as suggested at the link you provided?

>When Vista Aero theme is selected, windows get additional padding added (typically 4 pixels) which isn't accounted for in the built-in SYSMETRIC() fuction. I've tried calling the API GetSystemMetrics() function with the SM_CXPADDEDBORDER (IIRC it is 92) constant, but still get 0 as the result. The following indicates that it's the default behavior -- which doesn't help much.
>http://objectmix.com/clipper/749482-getsystemmetrics-sm_cxpaddedborder.html
>
>Anybody know of any way of querying this value? Yes, I'm aware that I could always kludge things and have a user-configurable value, but I'd much prefer to have my program determine this value by itself rather than relying on user to configure this item properly. This is affecting some code where positioning of various elements are now off because of the extra border (since this added border is on the inside of the window dimensions specified, this throws off the positioning of elements that I'm trying to place).