What antivirus / firewall did you use before avast?
Did you remove it before installing avast?
Did You use a removal tool to clear any leftover files that may conflict?
Check out this site - Uninstall possible remnants of previously installed AVs see, http://singularlabs.com/uninstallers/security-software/, this has a collection of manufactures removal tools, so that should remove any remnants, registry, etc.