Hello,
       I have installed an antivirus on my computer, but when I download any file from net the dangerous virusses also comes and damage the system, my antivirus fails to do so. I have fomatted my system many times, but nothig happen. So suggest me what to do.

   I would suggest two things: get an antivirus that works and do not download files with viruses.
   If you are the first person to be infected by a new virus, no antivirus program is going to help you.  The antivirus company has to have a chance to discover the virus, come up with a solution for the virus, and then distribute the solution to your antivirus program.  Some antivirus programs update their programs more frequently than others.  Some antivirus companies pursue virus solutions more vigorously than others.  Free antivirus programs are probably not going to update as often and so are generally not as good.
   Viruses are only in certain places.  If you avoid those places, you will have greater success at avoiding viruses.  Two such places that come to mind are email attachments and warez websites.
   Antivirus programs are usually good at finding viruses but they are not always as good at removing them from your computer.  If your antivirus program is detecting the virus but not removing it, do a search on the internet for the name of the virus.  Hopefully, that will turn up a solution that will work for you.

Quoted from ArielEdwardsLevy Hello,        I have installed an antivirus on my computer, but when I download any file from net the dangerous virusses also comes and damage the system, my antivirus fails to do so. I have fomatted my system many times, but nothig happen. So suggest me what to do.

There's a few things I would recommend you do, I use the free products myself and like them. ..I use AVAST 6 and its great. I would also say to install Comodo Firewall and Comodo Time Machine. The Firewall will help filter incomming files and if something does happen you also have the Time Machine that lets you revert your system back to before you got infected. It's nothing like the Windows Restore, as this program will completely revert it to s state as it was before. The after you have all these programs installed download and install Sandboxie. This is a very strong "layered" approach to protecting your computer. One last thing is to use a program like Macrium reflect free disk imagine software and image your system to an external or another drive.

Good Luck!