did you uninstall your previous used AV before installing avast?
did you run that vendors removal tool to clear any leftover files that may conflict?

see here http://www.avast.com/en-eu/faq.php?article=AVKB11#artTitle / http://singularlabs.com/uninstallers/security-software/

have you tried to repair avast?

controlpanel > ad/remove programs > avast > uninstall/change > repair option…