Um, ok. You need to provide info for us to help you. 🙂
What monitor do you have? The native resolution of a 23" ACD (or probably any other 23-24" monitor) is 1920x1200. Do you have your MP connected to a 1080p capable HDTV or something, with display mirroring turned on? Normally, 1920x1080 isn't a selectable resolution, but I've seen mirroring force screens into odd resolutions to fit to the lowest resolution of the two.