I would say a clean reinstall would be advisable.
Download the latest version of avast
http://www.avast.com/eng/download-avast-home.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
Download the avast! Uninstall Utility,
find it here and save it to your HDD.
Now uninstall (see below), reboot, run the uninstall utility, reboot, install the latest version, reboot.
It may be necessary to disable the avast! Self-Defence module (Program Settings, Troubleshooting section) if you are unable to uninstall using the Add Remove Programs before running the Uninstall Utility.
Or, boot into safe-mode and start aswClear.exe from there.