No network connectivity

After a period of inactive time trying to access the network or Internet the computer fails. If I try to ping the router by IP address the request times out. The only way I am able to make the computer work on the network is to reboot. It will continue to work unless the computer is inactive for 30 minutes or more. I have found that removing antivirus software McAfee and Norton makes the problem go away. I have tried two different versions of NAV and one of McAfee. IP addresses are statically configured and all Windows updates have been run. The pc has been completely rebuilt.