Emulating joystick programmatically
see my answer for a previous question that is really similar to yours.
Writing a windows driver for an emulated input device
Obviously it depends on what type of joystick input you are trying to emulate. Writing a custom driver is really something you want to avoid as much as you can. You may not need to do it at all.
Your best bet would be to modify the sample VHIDMINI - Sample for Virtual HID device from the Windows DDK. You could modify this sample driver to emulate a joystick HID device and then send out joystick HID reports from the driver.