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 highway for no reason. Accept this, restart and drive on.
Occasionally, executing a maneuver would cause the car to stop and fail to restart. You'd have to re-install the engine. For some strange reason, you'd just accept this too.
You could only have one person in the car at a time, unless you bought "Car 95" or "Car NT". 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 oil, engine, gas and alternator warning lights would be replaced with a single "General Car Fault" light.
People would get excited about "new" features in Microsoft cars, forgetting completely that they had been available in other cars for years.
We'd all have to switch to Microsoft gas and auto fluids.
New seats would force everyone to have the same size butt.
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.
Microsoft wouldn't build their own engines, but form a cartel with their engine suppliers. The engine would be a side-valve design so you could still use Model T Ford parts on it.
Microsoft cars would have a special radio/cassette player which would only be able to listen to Microsoft FM and play Microsoft cassettes.
Microsoft would do well, because even though they don't own any roads, all road manufacturers would give away Microsoft cars for free!
If you couldn't afford to buy a new car, you could borrow your friend's and copy it.
Whenever you bought a car, you would have to reorganize the ignition a few days until it worked.
You would need an upgrade to run cars on a highway next to each other.