NAV2004 gone from tray, won't re-start

I'm running Win2000, and NAV 2004 has worked fine for me since I installed
it several weeks ago.

Now, suddenly the NAV icon is gone from the tray, and when I try to start
NAV in "Programs", I get a little Installer box that offers to install it. I
OK on this (although I know that NAV is already installed), but the
installer quits after about a minute.

Task Manager shows that an application called navapsvc.exe is running (also
another app that starts with "sym", so possibly Symantec-related). And, I
was able to run the NAV "live update", or at least it appeared to run fine.

But, there seems to be a problem beyond the NAV icon just not appearing in
the tray. When I send outgoing email, for example, I no longer get a pop-up
telling me that the outgoing mail is being scanned.

Thinking that I might have managed to get a virus that is blocking NAV, I
downloaded the free version of AVG Anti-virus, updated it, and it scanned my
system, but no virus was found.

Any ideas what is going on here, or what I should do???

Thanks for any advice!

Arrggh! Here's the latest, much worse symptom.

When I tried to re-boot, I could only get as far as the Win2000 splash
screen. The "starting up" progress bar goes OK, and then....the screen goes
dark (it's a notebook).

The hard-drive light keeps blinking, and I can hear clicking, so the HD is
being accessed, or at least that's being attempted. But nothing else
happens.

Trying to boot into Safe mode gets this same result.

Any help, please??? Could be a hardware problem but, given the way it began
with the NAV going down, I'm thinking this is a virus, no?