Not long ago a friend of mine got an email with a malicious attachment. He had one of the most popular antivirus applications installed, but I don't remember which one. The antivirus didn't detect anything, and he ended up opening the attachment and only then he realized it was malicious. He sent it to me, and I uploaded it to a service like VirusTotal. The result was that none of the popular antivirus solutions were able to detect it, and only a couple of the less common ones (maybe Baidu Antivirus or something like that) actually reported it could have been malicious.
So antivirus software can definitely help, but it's not enough to prevent new kinds of malware (which is released continuously every day). Chances are that a few hours or days after new malware is discovered the antivirus databases are updated, but you are likely going to be vulnerable between the time of new malware release and the time when your antivirus is updated.
Antivirus software cannot guarantee that you are not infected, and cannot guarantee that they are able to get rid of infections completely. Once your computer is compromised the only real way to fix it is to format the disks and reinstall everything. When you restore data from backups you must be sure it's clean, for example because you stored it on an external hard drive that was never connected after the supposed time of infection, or for example because you have other ways to make sure it's clean (comparing checksums, etc.)