I have installed many anti-virus for protect my computer but no one it's working its good. i want to know how to come virus to my system somebody told me that its coming through USB and Internet and also i want know which one is better protect from virus ???

I have installed many anti-virus for protect my computer

How many A/V programs did you install? More than ONE is a bad idea due to potential conflicts.
Multiple resident A/V actually can make you LESS safe!

If you are looking for a good total Internet Security package and you don't mind spending some cash, I recommend http://www.kaspersky.com/kaspersky_internet_security

--- If you have already been infected with malware, I suggest you try the steps in my "Malware Cleaning Steps" linky below.

Cheers :)

Ok but one question is mine how virus reach my system?

You can try AVG, which is free along with spybot search & destroy. Once you keep them updated along with your firewall turned on and your updates from microsoft.....you'll be in decent shape. Post your hijack-log, if you want someone to check out your system for you (instructions at top of page). Pretty much, once you're connected to the internet, you're exposing yourself to potential viruses. If you use P2P programs or visit questionable sites, that also increases the chances of your system being infected. Definately, use only one anti-virus program & one malware program. Try my 2 suggestions (free programs).

I use Avira Antivirus
is it better for AV software

Install Zone alarm internet security,

and install avast antivirus

and when u place the usb to ur system scan first and then open

which virus is htere in ur system and what its behaviour post that in detail

Rakesh Nagekar