Where can I download Avast Secure Browser for Windows XP?
As far as I’m aware the current Avast Secure Browser (ASB) wasn’t compatible with XP. XP support ended some time ago end of December 2018, which predates ASB by some time.
There was an old version prior to the renaming to ASB, Avast SafeZone Browser, but I don’t believe that is available as an Avast download.
You might want to take a look at this big topic on the end of support for XP and see if there is anything there https://forum.avast.com/index.php?topic=220639.0. Or check out the Avast Secure Browser sub-forum as that also included posts on the old Avast SafeZone Browser.