MD5 hash wrong for many files downloaded

Avast Free Antivirus / Premium Security
1

If I download a ISO or Exe file in Win 8.1 with Avast, surely the MD5 hash file is wrong.
This happens with many Linux ISO images, Windows ISO from MS Technet or a simple PAF file from PortableApps.com and this happens with a direct download, using a torrent file or a download manager.

Instead, if I download the same file directly from a Linux partition I haven’t errors.

Is it possible that it is Avast to damage the files?

Thanks in advance.

2
Is it possible that it is Avast to damage the files?
One way to find out ..... right click avast tray icon and pause shields ....then try again