Hello,
I use a MacBook Pro with MacOS X 10.8.1 and NAV 12.3 (71).
Auto protect is sometimes off.
It's the same problem like here: http://community.norton.com/t5/Norton-for-Mac/Virus-Protection-off-Auto-Protect-could-not-load-scan-engine/m-p/784802/highlight/false#M6118
When I switch on I become the message "AntiVirus Auto-Protect could not load the scan engine. Please run LiveUpdate to get the latest version" too.
I try the hint with terminal and delete symantec.keychain. This works until I restart the MacBook.
Then I have the problem again.
Greetings,
Paul