@thejbforum. As DavidR mentions, best if you can post a screenshot or give more details of what you are trying to do and what preceeded the Error 500.
I still have Avast Free v18.5 on an XP/Linux dual-boot machine, but hardly ever booted as XP. Also being the Free version, it does not have Avast firewall, but uses the Windows firewall.
Edit: I booted XP with Avast 18.5 Free this afternoon without any issues. I had to wait for virus defs to be updated, which was expected, as last XP boot was many days previous.