PC's vs. Cars
Sorry, I don't think the PC can really be compared to a car.
Yes, you can go to a showroom and buy one pre-built (Apple are a good example of this), but that's pretty much where the analogy ends. I guess you could say they both take you on a journey, if you wanted to be particularly philosophical...
But the world of PC's is much different to a car. A car, for one, doesn't have an operating system. You can put in new parts without installing drivers. This is one of the downfalls of the modern PC. Each component needs software to drive it.
(Wouldn't it be better to have some sort of standardised driver set, that each device adhered to? This wouldn't limit the devices, just give them all standard ways to communicate with the OS.)
Updates to a car mean buying a new car (if you want the updated version). Upgrades to software don't mean you need to go and buy a new PC, although you may need parts.
Whatever way you look at it, a car and a PC will never compare - because a car doesn't have software. There's that whole other aspect to consider with a PC.