Even if it isn't supported you can usually use xpadder or joy2key to make each button on the controller simulate the key for what you want it to do. I play tons of old console games with my xbox controller.
It isn't quite the same on some modern games. You can get around 70% functionality but if the interface is designed around the mouse and keyboard it still feels off. For example, moving through menus in oblivion with a 360 controller required you to move the right joystick as if it were a mouse, instead of moving solely along the menu (like on the 360 version). Fallout 3 worked perfectly so I hope that trend continues...