You dont say what version of avast you are using but i would suggest a clean install of avast, download the free prerelease here and save as it has less issues http://files.avast.com/files/beta/6.0.1044/setup_av_free.exe, now uninstall your current avast from add remove programs and reboot then go to program files and program data and delet the alwil/avast files, now run ccleaner and your all set to install your new copy, if you are using a third party firewall make sure you add exclusions to it for avast and likewise to avast for your firewall, no exclusion’s necessary for the windows firewall.
Just in case you have not thoroughly cleaned any remnant’s of previous av’s you can use this link to find removal tool’s for any that you might of had http://uninstallers.blogspot.com/