what you are seeing is an automatic scan  and whilst it might be annoying when
you are working on your own programmes, which I hope are ok. I still would not
recommend turning the feature off.
to see what else is automatically scanned go to

Norton internet security---- personal firewall-------- configure----------
programmes.

to the turn off any programmes that Norton seeks to protect I feel would be
irresponsible ( I assume that you have never been infected with a virus) leave
well alone

Before I explain, let me first say that I HAVE been
infected by a virus, which is why I got Norton,
but I'm now clean... point being, I'm with you...
I don't want to turn off anything.

But, my confusion revolves about "who" is doing
the virus scan  when I load an Excel spreadsheet:

Norton or Microsoft?

Here's what threw me... your reply got me thinking
about Norton, so as an experiment, I disabled
AutoProtect as well as Script Blocking. I then
rebooted. I then launched Excel from my desktop
(i.e., a totally new, blank workbook). The phrase
"running virus scan..." still appeared at the bottom.
What gives? It's totally blank !!

Do you have a Personal Macro Library? What about other add-ins?

What do you see if you look at the task manager Spoo.. Ctrl Alt Del?
Do you see any norton or symantec programs running?

It took a few tries (it happened almost too
fast to read), but here's what happened:

Before loading Excel, 27 processes
Launch Excel... jumps to 29 processes, then
drops back to 28 processes.

The one that came and went was navw32.exe,
which is Norton. So that answers my first
question: Who is doing the virus scan  --> Norton.

I don't know what a Personal Macro Library
is, so, I guess I don't have one.

As for add-ins, one computer does not,
the second  does .. Bloomberg API. I notice
that on this second computer, an additional
virus scan  is performed on it as well.

1) start ----- control panel ------- security centre --- manage security
settings ----- windows firewall turn off

2) re-apply Auto as well as Script Blocking and any thing else you have changed
in Norton must be restarted'

3) re boot and do not tamper with Norton, make sure that you enable automatic
updates.

the security scan  is for your benefit and should be allowed to run

but there is no Security Center
on my Control Panel ... I've got Windows XP

Instead, it seems to be under Network Connections...
(click on the connection) ...
Network Tasks ...
Change settings of this connection ...
Advanced tab  ...
.. Internet Connection Firewall box was already empty.

Thanks to Lisa's suggestion, I've determined that
it is indeed Norton doing the scan.

Norton seems to be less "advanced" than Microsoft,
in that there is no "Trusted Publisher" feature
in Norton. One can only select file types (i.e. ".XLS")
to be eliminated from the scan, and of course I don't
want to remove that for cases where I receive .XLS
files from outside sources.

So, all Norton defaults are enabled, and I will
put up and shut up.