Norton Antivirus does require a regedit after an uninstall. So:
1) Uninstall NAV
2) Reboot
3) Run regedit
4) Search for Symanytec
5) Delete all instances of Symantec (if you have other symantec products, it's best to uninstall them as well and delete them from the registry at this step too).
6) Reboot
7) Install McAfee
8) Then reinstall your "other" Symantec products.
My experience with NAV is that it's not removed from the registry and must be removed manually. Also NAVputs a shortcut in Startup folder for NAV AutoProtect... you need to manually remove that shortcut as well.