Just got hacked. But how?

Ok... Just bought Norton after years of enjoying FREE AVG and just got hacked. The date in my system was changed, which made it seem like the subscription for Norton had expired, and the only thing that saved me was my browser giving me an alert that the site I was about to enter (Google of all places) was unlikely Google and that my system was not protected.

How did this happen? How do I know it won't happen again? I was under the impression that Norton was supposed to protect me from little things like this. I am confused.