Can someone still hack my laptop connected wirelessly on the internet even thou I have an antivirus?

I recently purchased a router so I can access the internet whenever I want in any parts of the house.When I Installed it says unsecured network how can I put a pword on it so no one can access on it except and they have to enter a pword? can someone still hack my computer even thou I have an antivirus on I'm using windows live one care....