The Avast uninstall utility is a simple program called avastclear.exe.
Follow these steps in order to uninstall Avast from your computer.
Download Avast uninstall utility from here. Save it on the desktop.
How to uninstall Avast, using the uninstall utility?
Start Windows in Safe Mode. For Windows 7: hit the F8 button from the point where your computer boots.
If you do it right, you should see an option to enter Safe Mode.
It works the same for Windows XP and Windows 8.
Once in Safe Mode, run avastclear.exe.
Click the “next” buttons and “remove”.
Restart your PC.
Avast should be fully removed now. You can download and install the latest version.
Usually, users can’t upgrade to the latest version and also can’t uninstall Avast. That’s why the uninstall utility is required in this case.