Avast secure browser is the successor of Avast Safe Zone and it's a fork from the chromium repository modified by avast. Chromium dropped support for XP time ago and as result, Avast secure browser dropped it too and is compatible with Windows 7 and up, which is fine.
What's weird, though, is that after I installed the latest beta, every time I boot my computer an error message pops up saying that Avast couldn't find a directory in app data in which there should be "avast secure browser checker.exe". I personally checked the directory and the file is there. Out of curiosity, I tried to run it, but it didn't do anything.
I don't know what Avast is trying to do, but whatever it is, I think it should be disabled if it detects that is Windows XP, in order to avoid to show the error.
OS: Windows XP Professional x86 SP3