I just down loaded and Installed Avast Free
Did you remove previous used AV using vendors removal tool before installing avast?

General: Uninstalling a third-party antivirus software https://www.avast.com/faq.php?article=AVKB11#artTitle

Have you tried several reboots?

Have you tried avast repair? Controlpanel > ad / remove programs > avast > uninstall / change > repair option