The explanation was there was a conflict with Windows Updates and earlier versions of Avast, so those users were automatically updated.

Several threads on this topic, and a mea culpa from Avast that users should have received advanced warning.

That said, you can roll back to the previous version 9.0.2008.

Older versions of Avast can be downloaded at Filehorse.com.

http://www.filehorse.com/download-avast-antivirus/old-versions/

I would recommend running the Avast Uninstall Utiltity, just to make sure all bits are off your system.

http://www.avast.com/en-us/uninstall-utility