The older avast installer contains an "expiration date test": if it's "too old", it won't install, and you get a message that it'll proceed to get the latest build from the website.
If you're not satisfied with the latest build (for whatever reason), and prefer to use that old build instead:
a - reset the PC's calendar to a date when that build was still "current" --- for 1426, I believe that would be about late-March 2012.
b - Disconnect your PC from the internet, so as to be sure this old build wouldn't immediately search for and download any newer builds.
c - run the old installer.
d - when the installation was completed, open the avast interface to make sure that PROGRAM updates are set to MANUAL.
e - finally, reconnect your PC to the internet, so that avast would find the latest DEFINITION updates.
Note: If you've customized your avast settings, be sure to back them up first, so you can easily restore them afterwards: avast interface, SETTINGS (upper-right), Maintenance tab, scroll down to SETTINGS BACKUP.