are you running vista? mine does this on GTA:SA and to fix it i go into options>controls> then change from "mouse and keyboard" to "joypad" then back to "mouse and keyboard" again. It seems to kind of reset it so it recognises the input devices. then go back to game and the mouse should work. (you will need to use the return butten and arrows to naviagte menus until you get the mouse working)