NAVENG and NAVEX15 are components of the antivirus engine used in Symantec Endpoint Protection. Issues with these device drivers are often caused by corrupt or otherwise invalid definitions. In the event that you are running a managed environment (Definitions are distributed to client machines from an Endpoint Protection Manager) you will need to run the Support Tool on other clients, to see if the issue is present across your environment.
To determine if a client is self-managed, or managed:
If the Endpoint Protection icon in the system tray has a green dot, then it is a managed client. Otherwise, double click on the icon, and in the upper right corner select "Help and Support", and then "Troubleshooting". Underneath General Information, it will list the hostname or IP address of the Management server (if present) if stand-alone, it will say "Self-managed"
ReferencesHow to update virus definition files using the Intelligent Updater