Surely you still have the copy that you first downloaded, unless that was corrupt and you had corruption errors (you never mentioned what the problem was) then you can reuse that.
Up until just over two months ago I was on dial-up (only then was I able to get a slow broadband service) and that was what I always did download the new version and save it so it can be used again if required. I also usually have a copy of the avast uninstall utility squirrelled away to, so I can do a clean reinstall. Download the avast! Uninstall Utility,
find it here ensure you get the version5 one, aswClear5.exe and save it to your HDD.
Above all else if you can't get broadband, find a friend with broadband or local library, etc. so if you do have to go on-line where on dial-up it would take hours, ask them or use their service and save the file to a USB stick.