If I write a new virus today tell me how your anti-virus will protect you against it.
Nowadays, most decent AV packages have firewall and proactive features that not only look for particular viruses, but virus-like activity as well. So, it's very possible that an existing AV package will catch a previously unknown virus before infection.
Of course some of the better viruses are written and tested to ensure they circumvent most major AV packages, so sometimes your particular AV will not help.