They report that in some games when the user selects 4xAA, the drivers enable 16xAA instead. This results in better image quality but significantly reduces the framerate.
To set standard 4xAA in these applications, set 4xAA in the game, and also enable “Enhance the application” anti-aliasing mode with a 4x anti-aliasing setting in the NVIDIA graphics driver control panel. NVIDIA is working with developers to implement better in-game CSAA support. You can see CSAA menu selections in Half-Life 2: Episode One and Supreme Commander.Some of the affected games: Battlefield 2, Battlefield 2142, Sin Episodes, Half-Life 2 and Half-Life 2: Lost Coast.