I don't have anti virus in my system but it is safe until now. I don't have a firewall system also. is there chances to attack virus on my system?

Most people need an antivirus. You never can tell when you will get one over the world wide web. You can get them from email attachments that you open, programs that you download, and when you visit a site on the web (via a drive-by download). There are several free AV programs that provide good, basic (realtime) AV protection--like Microsoft Security Essentials, Avast, Avira AntiVir, Panda Free Cloud, and Immunet Free. You need a real-time AV that protects you while you surf. ClamWin is not a realtime scanner--it is an "on-demand" scanner that does not protect you until you do a manual or a scheduled scan. It is a good backup AV that does not interere with a realtime AV.

Regards,

I agree that AV is important in all internet user because virus is always a treat in all internet user. However, I wonder how many times in a day does an AV have to update?

An AV should update its virus signatures at least once a day, but viruses are being created all the time, so it is probably better for an AV to update signatures several times a day.

As for updating the AV program for improvements in detection, there should be one yearly update at least.

Regards,