Just thought I'd share how I finally got rid of the bits my Norton Antivirus had left behind in the registry. There were numerous Symantec entries in every section of the registry keys.
Nortons was well and truly gone to all appearances, didn't even leave bits of itself in the All Programs menu. Then I was checking something out in my Add/Remove Programs in the Control Panel and noticed a couple of Symantec Live-Update items still installed!
I ran the Program Files uninstaller on them and now all Symantec programs are completely gone from my system.
I know people often reinstall completely to get rid of Nortons and I hope no-one minds my posting this experience - it might help someone out because I found it wasn't necessary to do that and it's a much less scary fix for most people than editing the registry and far less painful than a reinstall!

I'm running XP Service Pack 1 so this may not work on all computers but it's worth trying.
On the other hand I still have a few Zonelabs registry keys in the Software sections, should I just delete these? I no longer have any Zone Alarm software on my system, they're just dead registry items.