Every time they repainted the lines on the road, you'd have to buy a new car.
Occasionally your car would just die on the motorway for no reason, and you'd have to restart it. For some strange reason, you'd just accept this, restart and drive on.
Occasionally, executing a maneuver would would cause your car to stop and fail to restart and you'd have to reinstall the engine. For some strange reason you'd accept this too.
You could only have one person in the car at a time, unless you bought a "Car 98" or a "Car XP". But then you'd have to buy more seats.
Macintosh would make a car that was powered by the sun, was twice as reliable, five times as fast, twice as easy to drive - but it would only run on five percent of the roads.
The Macintosh car owners would get expensive Microsoft upgrades to their cars which would make their cars go much slower.
The oil, engine, gas and alternator warning lights would be replaced with a single "General Car Fault" warning light.
People would get excited about the "new" features in Microsoft cars, forgetting completely that they had been available in other cars for many years.
We'd all have to switch to Microsoft fuel and all auto fluids but the packaging would be superb.
The airbag system would say "Are you sure?" before going off.
If you were involved in a crash, you would have no idea what happened.