Security software sometimes may not uninstall properly on a system it is installed on. It can be that the default uninstaller that ships with the program is broken and thus non-functional, or that it leaves traces behind that prevent the installation of an update, other version or third party product on the system.
The majority of companies that produce security software for Windows make available removal tools that delete all traces of a product from a system. These tools use hard coded lists of files and locations usually and are considered thorough because of it. Sometimes, they combine the list with searches to make sure that all program files and Registry information are removed from a computer system.
The Avast! Cleanup Tool is a third party program that is not to be confused with the company’s official removal program Avast! Uninstall Tool.
Why you’d use a third party program when there is a first party program available for the same task? Because you do not have to run it in safe mode which you need to do when you use the official Avast utility. It may also be your only hope if the official program for one reason or the other does not work properly.
First thing you need to make sure is that it is supporting the version of Avast! that you have installed on your system. At the time of writing, it is supporting the following versions:
  • Avast! Antivirus 7.x Free/Professional/Internet Security
  • Avast! Antivirus 8.x Free/Professional/Internet Security/Premier
You should not use the program if you are running a different Avast product or version of a supported product. To completely uninstall a supported version of Avast, simply run the tool after you have downloaded it to your system. First screen displays information about the program and its intended uses. Once you click on accept, you will notice that two command line prompts are launched that perform the uninstallation of Avast software on the system.

Please note that the cleanup tool is only compatible with Windows Vista or newer versions of Windows, and not Windows XP.
The program can be useful if you need to remove supported Avast software from a Windows system and can’t or do not want to use the official uninstaller to do so.
It is highly recommended to try the official Avast tool first before you make use of third party software.