Thank you for your responses.
Have xp, 56 modem.
Downloaded from official site.
Was registered with avast but hadn't used the laptop for a long time and registration expired. When I tried to renew registration, was told my version was too old and had to download 4.
This is when the trouble began.
No, not using download manager.
After several tries of downloading, I deleted all avast files on my machine and downloaded again. That didn't help, either.