The easiest and quickest way would probably be to use the Arduino (item #2 in that spreadsheet on page 1); but that is only part of the hardware.
Unless the PC is going to be mounted on the car (and in that case, unless you are using something like a BeagleBoard, you're going to need a larger chassis - which can change things dramatically depending on the size), you would need some form of wireless communications from the PC to the Arduino; there are plenty of wireless solutions - a popular method is pair of xbee transceivers (one is hooked to the PC via serial, the other to the Arduino via either hardware or software serial).
You haven't said anything about the "car"? Is this car pre-built? If you've never built a robot chassis from the ground up, I would recommend going with something pre-built, or at least a kit. If you wanted to do it the "super simple" way, get a cheap hobby R/C car (something that uses real servos), then set up the Arduino with the Servo library to control them. If you are planning on building your own chassis, though, you will need motors, something to control the motors (motor controllers, h-bridge, etc) - plus you may need a servo or something similar for steering (unless you are going to use differential steering).
Actually - we need to take a step back here - answer these questions first:
1. Are you building or buying the chassis?
2. Will it have differential (skid) steering or Ackerman (car) steering?
3. How many motors do you envision it using?
4. Will the PC be mounted to the chassis, or will it be remotely connected (via wireless).
Regardless of the answers to the above - sticking with the Arduino will get you running the quickest, IMHO.