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 software do you have installed, including firewall ?
How are you actually downloading this file, download manager, from a browser and https link, P2P (if so what client) or other means (whatever that might be) ?
During the download is the avast icon rotating (especially if it is indicating 100% cpu) ?
Try opening the avastUI and Real-time Shields, File System Shield so that you can monitor any activity, and try starting the download again and see if you can see what is actually being scanned ?