Try an update of your NAV program by manually running LiveUpdate, or if that doesn't help you'll need to disable Norton's email scanning.
If you're concerned about disabling email scanning read this:
"Note: Disabling email scanning does not leave you unprotected against viruses that are distributed as email attachments. NAV Auto-Protect will scan any incoming files, including email, as they are saved to your hard drive. Email scanning is just another layer on top of this. To make sure that Auto-Protect is providing the maximum protection, keep Auto-Protect enabled and run LiveUpdate regularly to ensure that you have the most recent virus definitions."
As a last resort you could try an uninstall/reinstall of NAV - but I personally wouldn't recommend it because of the hassle involved & potential problems you may have if it doesn't install correctly.