firefox is not working, new installed

I installed a firefox, but i found the firefox is not working. Even i turned off the zonealarm. no aother firewall installed. anyone know this problem. this is not a new lap. ms ie is ok for some pages, that means the hardware and net is fine. all other pc in the net is work. I use kav5.0.527 personal pro to scan, no virus found. But i still think there is virus exist. no 3721 virus found.

any one can find the problem.

thanks.