I think you are getting some terminology confused, drivers are not stand alone software applications. Some drivers do come with a driver utility program, however drivers are meant to provide the software layer between a physical piece of hardware connected to your computer to the software which controls this hardware. It is basically an in between piece of software you cannot directly interact with as its only role is to let the controlling software and hardware communicate. The controlling software in this case would be your operating system. So those drivers get installed to the same drive your operating system is installed, otherwise the operating system can't communicate with the hardware, which in your case is your video card.
Autocad is a stand alone software application. When there is no hardware involved on this level, the applications can be stored on most any local storage device.
