Today, users are taking advantage of cloud-based applications that simply require an internet connection to get their work done. Many people think anti-virus acts as a wall of protection between us and malware on the internet. But that isn't the case.
When you rely on anti-virus, you are only protected from known types of malware. Every day tens of thousands of new strains of malware are released, known as zero-day attacks, which leave antivirus companies scrambling to develop a defense. By the time you're properly defended, hackers have probably moved on to the next new thing.
If you're still periodically scanning for vulnerabilities on your network and don't have a handle on dynamic assets, you can't give a clear answer.
While your network (firewall) and endpoint defenses (anti-virus) react to malicious activity. SecurityEssentials utilizes security tools that prevent and detect attacks before they happen.