Yes, you're right. However, starting with version 4.0.172, there actually IS such a functionality in avast! Setup - it should warn you if it finds Norton Antivirus installed on the target machine...
Are you saying that mere uninstallation of Norton is not enough? I.e. that a special (more thorough) removal tool is needed??
We'd be grateful if you could provide us with as much information about this issue as possible.
Thanks,
Vlk