@CybSnoopy,

After uninstalling BitDefender and before installing Avast, have you done a reboot?

You could try repairing Avast from the “Add/Remove…” Control Panel.

If that doesn’t work, download again the setup.exe of Avast and the Clean Uninstall tool from http://www.avast.com/uninstall-utility and run that uninstall utility from Windows Safe Mode.

Once uninstalled, reboot into Normal Mode, close any unneeded programs and run the downloaded Avast setup.exe (the new one, don’t use the previous one). Once the installation finished, reboot.

Please for the next time start your own topic.