cannot get avast to show up in the task bar

I had viruses and was unprotected. I recently had the computer completely cleaned up from viruses. But now I want to install Avast Home Edition and it seems like it won’t install correctly. I have uninstalled and repair a few times already. The same thing happens with any anti-virus. Is there anything I can do?

What is the operating system, please, and what other antiviruses have you tried to install?
What actually happens when you try to download and install Avast?
How do you know for sure that the computer is clean?

Welcome to the forum.