hi…2 questions please…#1 can i install the avast browser using windows vista, and #2 can i install avast browser without using the avast antivirus??
NO. You need a min. of Windows 7 to install the Secure Browser.
→ https://forum.avast.com/index.php?topic=223368.msg1484702#msg1484702
Repeating the question won’t change the answer. 
Maybe Avast needs to make up it’s mind what requirement are needed:
https://screencast-o-matic.com/screenshots/u/Lh/1543158922008-6814.png
https://www.avast.com/en-us/secure-browser
When you look up that information under the Facts section, you get a different answer.
I haven’t used XP in many years and can’t personally tell you if it works on that operating system or not.
It won’t.