Why Using Multiple Antivirus Programs is a Bad Idea http://blog.kaspersky.com/multiple-antivirus-programs-bad-idea/ https://blog.avast.com/2014/05/09/when-software-collides-what-to-do-with-your-old-antivirus-program/
are you able to start your comp in safe mode? avast Uninstall Utility http://www.avast.com/en-eu/uninstall-utility
General: Uninstalling a third-party antivirus software http://www.avast.com/en-eu/faq.php?article=AVKB11#artTitle