Well, it seems that avast! simply doesn’t support the format of the Nero installer.
I just checked the file and it’s a WinRAR archive with a custom SFX header - which is quite long. So, the actual RAR file is appended much farther than it is usually…
I’ll try to do something about it - but the fact is that the bigger part of the file will be checked by avast! for RAR header, the slower the scanning will be.