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