"Windows 7" is a bit too vague. If you've got Windows 7 with all updates (let's say at least before the paid/extended support kicked in), then I believe latest Avast should work [unless you've got a really old CPU not supporting SSE2 instruction set, or something like that). However, a really old Windows 7, e.g. without SP1 or without SHA-2 signing update (there may be other required updates to be installed, I honestly don't know), that won't work.
I suggest to post a log from the unsuccessful installation, maybe it has some useful information.