Forgot to add, after you remove Avira with regular uninstall it would be wise to also run the Avira RegistryCleaner, that should completely get rid off all registry traces of Avira.

Avira RegistryCleaner - http://www.avira.com/en/support-download-avira-registrycleaner

In case you need them, you can find removal tools for all other AV’s here:
http://thewebatom.net/uninstallers/security-software/