I have Windows xp home sp2. I have been using Norton anti-virus 2006
with no problem for several months.  After running live update Feb 24,
and installing the updates it suddenly started refusing to scan some
files (like .exe and .dll).  I have highest security set and no
exceptions.  I tried uninstalling and reinstalling the entire program,
but when I ran live update and installed the latest updates it quit
working right again.  Also, when I use the Symantec automatic
assistant, it tells me I don't have the latest version of live update.
 I let it install what it thought I needed, but when I ran the
asistant again I got the same message.  The activity log shows that it
scanned files, but the scan complete window says 0 files scanned. 
Also it used to take about an hour and 20 minutes to run a full system
scan, and now it does it in about 45 minutes.

sometimes you can get a virus that will disable your antivirus. so go
to this link and have a free online virus scan done.
   www.bitdefender.com/scan8/ie.html
if it doesn't pick up a virus, then its a software problem. since
youve already tried reinstalling it, try taking it to a technician or
something. if it did pop up with a virus, go into safe mode(directions
below) and run norton antivirus within safe mode. and you could even
try reinstalling norton antivirus in safe mode and running it.. if
none of that works taking the virus out goto Google and search the
virus and see if they have removal instructions. if that doesnt work,
then take it to a technician. I hope this helps you with your issue,
good luck!!

**To get into the Windows 2000 / XP Safe mode, as the computer is
booting press and hold your "F8 Key" which should bring up the
"Windows Advanced Options Menu" as shown below. Use your arrow keys to
move to "Safe Mode" and press your Enter key.