Hi
I'll be the first to say you don't need to install anything - honest. There are apps available, but as you've discovered, the recommendation is to avoid Norton like the plague. Intego offer Internet Security Barrier, but you don't need it.
If you feel you must install something many on here, me included, use clamXav which you can download for free although the developer rightly asks for a contribution, and just run it from time to time for peace of mind.
The best protection is to make sure that you are careful on the web and only download from bona fide sites.
There is an interesting link on the Apple site that goes into greater detail about internet security - if I can find it quickly I'll post back and add the link.
Just enjoy your new iMac when you get it.
Here's the link:
http://www.apple.com/macosx/security/
Message was edited to add link