Actually, there is something out there that does turn NAV off. When it happened to me, i believe i went to the norton website and found out what to do.
Its either a virus or spyware that does this, and i think its a virus, pretty sure. But again, you can find it at nortons website.
I will see if i saved any info on that when it happened to me. Sorry i cant remember more, it was a while back.
And it is true, NAV gets turned off and will not work at all until you fix it, the fix wasnt too much work, i just cant remember what i did.