Well, I took a deep breath and pulled the plug on Norton...
Thanks to the help and advice in this forum it was a fairly painless procedure (mind you, being ever cautious I did set a restore point after each step!) and everything seems to be working as it should!
For the record, here's how I got rid of NIS2002...
Ran RegMechanic and cleared up any existing problems and made sure the registry was tidy.
Used Control Panel to uninstall NIS2002
Reboot
UsedControl Panel to uninstall LiveReg
UsedControl Panel to uninstall LiveUpdate
Reboot
Used the Symantec utility RNav.exe to mop-up NAV
Used the Symantec utility RnisUPG.exe to mop-up NIS
Used the Avast external control to mop-up any left-overs!
Rebooted and installed Avast!
Ran the 'new' Mail Wizard 'ashPopwz_2.exe' referred to in the above post.
Rebooted and ran RegMechanic to clear up any remaing Symantec entries (and, yes, there were several still lurking!)
.. anyway, all seems OK and I'm a happy Avast user.
Thanks!