This is a very specific tool that have been designed to be used to get rid of, and in the case of avast! Antivirus Free and Professional, or avast! Internet Security.
When you have indicated that you want to remove a program, it stands to reason that you would prefer that no traces of it were left behind, and this is something that Avast Clear (Uninstall Utility) can help you with. There are occasions when an uninstaller files for one reason or another, and this could mean that software you do not want is left untouched on your hard drive, or that remnants of it are left behind.
In most cases you will find that this gets rid of the program in question in its entirety, but this is not necessarily the case. If you have decided that you no longer want to use a particular program, you may well head to the Control Panel or the Start menu and run through the installer.
