OK, is it showing a file within the installation file, if so what ?
Check the avast! Log Viewer (right click the avast 'a' icon), Warning section, this contains information on all avast detections. C:\Program Files\Alwil Software\Avast4\ashLogV.exe
If it does you could pause the web shield to at least allow you to download it, the standard shield would still alert when the download completes, select no action (leaves it in the downloaded location.
I use 7zip and that can open .exe installers this would allow for the suspect file to be extracted, to the suspect folder mentioned earlier. Then that could be checked and or submitted.
Or you could still send the report to email@example.com
, with a link to the download location and this topic might help and possible false positive in the subject.