You have to compare what others reporting an issue have, is their OS and avast program version the same. In your case with win7 and avast 17.9 do they have the same.
If this were truly a major issue (with hundreds of millions of avast users) this forum would be red hot.
I was making some investigation and it could be possible that the problem appears when avast tries to make the rootkits analysis on startup in old versions.
It could be possible that avast makes some changes for the new version on the defense module and mainly on the rootkits analysis and those changes affected old versions.
I can confirm that on another computer with the same operating system and the same version 17.9.2322 happens the same problem.
Is it time to try a more recent version !