If you run Win 7 or Win 8, you could try disabling the "Windows Defender" which is pretty much running invisibly in the background.
For Win 7 - follow these steps:
This is done by first going to Control Panel, find Windows Defender, turn it off. Then run MSconfig and
find "Windows Defender" among the "Services", and disable that. If you can't find it, keep searching.
Then restart your computer.
Then try the Avast install again. If it doesn't work, then forgetaboutit, plus remember to re-enable "Windows Defender" in the MSConfig Services, and as well in the Control Panel, otherwise you got zero defenses going on. And restart.
Always use garage logic: If you take something apart, line up the parts in sequence, and don't lose any parts. If you put it back together and you got parts left over, the odds are you missed something.
For Win 8: Ask someone who understands Win 8.