Norton virus protection shutsdown - virus?

Just recently I have noticed that my Norton 2004 virus program suddenly
disables itself. What would cause this? I have Windows XP Service Pack 2
installed. Would the windows firewall be shutting down my NV protection? Or
could it be possible that I have a hidden virus on my computer that Norton
hasn't found?

I browse the internet, read email, and newsgroups alot, so I'm very
concerned when I noticed that my virus protection is disabled.

I'm the only one that uses this computer, so there is no way that anyone
can disable the virus protection.

Should I be alarmed? Or is this some kind of quirk of Norton 2004 virus
protection?

 

When it is shut down, there ain't no protection.
Perhaps the virus that did the dirt is SP2...

 

I came across some virus (such as Sobig virus) capable to disable your
antivirus from Windows' startup.

Try to boot to safe mode and run NAV and do a full virus scan to you
HD(s).

p/s make sure you are having the updated/latest virus data file.