My windows machine got infected before I installed any anti-virus, and now I cannot open any file, including AVAST icon.
How can I install AVAST now?
Tks!
Meneghel
Welcome to the forum! 
Perhaps an Online-Scanner can help you at the moment.
Try it.
When your system is clean after that, then you can try to install avast!.
But perhaps it’s better to install windows new to have a clean system from the beginning…
+1 Download Avast and save it in flash disk or something (make sure this media is clean). Reinstall your OS. Install Avast in this clean OS before you connected to internet. Sometimes it’s easier to reinstall your OS than dealing with system repair.