Having two or more antiviruses running in the same time on the same machine is not the best idea... of course if both resident protection are enabled. avast! is more than enough, so you won't regret if you switch to avast! It doesn't matter if you disabled McAffee antivirus, it's traces are still present in your computer.
Prior installing avast! you should completely uninstall other antivirus and completely wipe out all traces of previous antivirus. Otherwise, you may have all kind of problems, not just if you install avast!, but any other antivirus. Same thing is with firewalls. Never have more than one firewall installed on one system. Properly configured, even one firewall is more than enough.