For sure, this is the reason of the avast problem, I mean, being deactivated.
How did you uninstall Avira? I know that there is a tool for complete uninstall it… I should search a little…
Avira AntiVir RegistryCleaner: http://www.avira.com/en/support/support_downloads.html
But I’m not sure it will remove AntiVir version 7, only 6 (the free one). Which one were you using?