rightclick on desktop, properties, settings, advanced, the outer right tab there (with nvidia cards it gives the name of your card) try some of the menus. most likely it'll be a slider that you can turn to the left for off.

antialiasing is making the corners of a 3D object sharper, by rendering it in higher resolution than the rest of the game.
eg: you play in 1024*768, but the corners of a model are rendered in 1600*1200 wich makes it look much smoother.