An IR LED and IR phototransistor will do for the car detection.
http://home.cogeco.ca/~rpaisley4/ATDetLong.GIF

You can look on http://www.ftdichip.com/ for creating a parallel or serial to USB interface.

So you have no electronics background so you're looking for off the shelf components so as to build your project. You want to do sensor monitoring but keep focusing on a camera, which you really don't need. You in essence need to detect existence of a vehicle. The camera will be good for detecting motion. Have you investigated various security equipment available? Get an Ultrasonic detector and hook it up to a GPIO board, which is linked to your PC. The GPIO board can monitor multiple sensors, not just one. The only electronics required then would be wiring the harnesses. Though you could build an electronic circuit using a PIC but you can find the embedded board. Try www.digikey.com or www.mouser.com or a company like that!

To send the data from the sensors to a PC. Isn't that what you want to do?