It is possible to get hacked even if you do have security, but it is not LIKELY unless you make yourself a target. I've had my computer crashed multiple times by online game opponents. I have even had somebody launch a DDOS attack on me once which lasted for about 4 hours. When I ran a game server, we pissed off a few cheaters by banning them from the server. They responded by hacking the linux box repeatedly.

I had quite a bit of security in all those instances. Nothing is infallable. However, like your car, people are unlikely to break your lock/windows unless they have an incentive.

-Biscuits