antivirus
 

i have fedora 4 and 8 install on each system need any antivirus if yes please give me url for antivirus

You don't need any antivirus if you don't have any virus.
If you want one anyway, use clamav.
yum install clamav
When virii are there, it's already too late.
You should use a pro-active aproach instead: don't use the internet as root, update your system with security updates often, don't open suspect emails and generaly don't do stupid things like giving your password on irc or posting your ip on a cracker forum and you should not need any antivirus.
Actually very few people use any antivirus on Fedora.

I would suggest you search LQ for "what Anti Virus". this will give you a bunch of answers to this question.

If you feel you need to run a virus program clamav is the open source one available. It suppose to be good.

I would also consider a root kit scanner. Your bigest risk is if you don't keep your system updated and someone hacks it they will put a root kit on and take control of your computer. So your best security bet is to keep your system updated and run a root kit scanner on a regular basis.

The two RK scanners that i know of are chkrootkit and rkhunter.