1. Download avast! Free Antivirus: http://files.avast.com/iavs9x/avast_free_antivirus_setup.exe
2. Follow instructions: http://www.avast.com/uninstall-utility (Run this tool for all prior installed avast! versions..!!)
3. Reinstall avast! with the downloaded installer from point 1.
4. Reboot.