does it mandatory?

Your computer, if it has Windows OS7 or OS 8, has some built-in anti-virus protection. so having a different anti-virus software such a 3rd-party (Norton, McAfee, AVG) instead, would depend on your desire to further reduce your computer to constant security risks. If it's your own home system, you can do anything you want, of course. In a working environment it is probably mandatory.

That's great to know. Never knew there is already antivirus in these OS. Wonder how my system got infected so many times even after using Avast. I think it is always better to keep cleaning your system and keep updating your antivirus. Why one's system gets infected could depend on how it's used, not necessarily what antivirus you use.

I believe it's called Microsoft Security Essentials. It comes with the OS, but as I recall, you have to set it up yourself. That could explain your issues.