In Grand Theft Auto: San Andreas, for the PS2, you seemed to have an option to automatically aim at people. That or it was default, but alongside it you could manually aim. In the PC version, I can't seem to auto aim, only manually aiming. Is there any way to turn this on?
The only way to auto aim in a PC version is to use a joypad/controller setup instead of the keyboard/mouse setup. You can set the controller configuration to joypad and use the keyboard for it, but then you won't be able to use the mouse.
There is a mod here: Automatic Weapon Aiming, that claims that it will "eliminate the need to hold the right mouse button in order to aim weapons, this is almost the same as automatic aiming." I don't know if it works or how well it works, as I have not tried it, but it might be of use to you.