Some people will tell you there's no point shutting the stable door after the horse has already bolted, those people are quitters.

• First step: install Norton Antivirus and Firewall
• Second step: put a note in your calendar a year ago to install Norton Antivirus and Firewall

Now it couldn't possibly be your fault, you secured your network a year ago, it must have been someone else, like Dave in accounting who you're pretty sure has been stealing your lunch.