Unfortunately for a solution, we have to find what is causing the problem (corruption/file size restriction) with the downloaded file.

  1. Normally a file during download would have a temporary file name like filename.part in firefox, etc. and only when the download is complete, does that change to filename.exe.

Obviously if the file you are downloading is bigger than the file on the system it won’t be complete, hence corruption as they are incomplete. But as I said there is no download file size restriction in avast, so why this is happening I simply don’t know. A download manager might well help if for no other reason it should allow for restarting the download should the connection drop out or fail, etc.

  1. Is it the web shield that you are stopping or all shields ?
    If all shields try just the web shield and see if that works.

Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?

  1. Do you have some locations of files that failed to download ?