Really? I've got winXP and I took a look at the windows task manager and the Norton threads only use around 4 mb, is that bad?
The amount it uses depends when you look.

The problem is that as files are loaded in, they get scanned assuming you have real time scanning (which recent Norton AV software does). This happens constantly in the background and can have a detrimental effect on the appearance of the dreaded "blurries".
At these times, not only is extra RAM used by the antivirus software, but of course the files don't load as fast, which can slow the update of scenery around you.
I've never measured this, so it may or may not be a significant amount of time difference, but bearing in mind that the RAM available to store the files at the point of loading is reduced by the antivirus software, it happens at the worst possible time from an FS2002 point of view.