I get the window after system boot if internet connection is disconnected/not available.
I think its related to Avast coz the window appears & Avast splash icon appears on taskbar.
If I click on Avast splash icon the window disappears.
If I click anywhere then too the window disappears.
What previous antivirus did you have? Did you use it’s removal tool in safe mode after uninstallation via add/remove? You should have done a repair of avast via add/remove and follow the instructions and after that done a manual restart.
If issue comes up again. Do this following?
The previous antivirus you used? Download it’s uninstall utility and run it in safe mode.
Uninstall avast from add/remove and follow the instructions.
Download and run the avast uninstallation utility and allow this tool to run your computer automatically safe mode.
Download and install a fresh copy of avast. Install avast in custom installation so you have the power to choose which feature you want to install. Once installation it will do a quick start up scan. Once finished, restart your computer manually.