Sometimes it´s not possible to uninstall avast! the standard way - using the ADD/REMOVE PROGRAMS in control panel. In this case, you can use their uninstallation utility aswClear.

How to uninstall our Avast using aswClear:

Download aswclear.exe on your desktop

Start Windows in Safe Mode (reboot, tap f8)

Open (execute) the uninstall utility

If you installed avast! in a different folder than the default, browse for it. (Note: Be careful! The content of any folder you choose will be deleted!)

Click REMOVE

Restart your computer