Because the main Avast download page typically serves the newest version (which will not run on ), you must use the correct installer. Discontinued Avast support for Windows XP and Vista - FAQs

While was once a staple for Windows XP users, official support for the operating system ended several years ago. You can still download and install a specific older version of Avast on a legacy Windows XP machine, but it will no longer receive program updates or new security features. The Last Compatible Version: Avast 18.8

: As of early 2026, Avast continues to provide virus definition updates for version 18.8 to help identify modern threats, though these are no longer tested on XP and are used at your own risk.