I can’t turn it on, can’t open it, can’t uninstall and can’t download a new program. What can I do?
Perform a clean installation of avast.
Use aswclear to remove things from avast.
Install the latest version.
Why can’t you do those things, e.g. what errors are displayed ?
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
What other security based applications do you have installed, firewall, anti-malware, etc. ?