Well if you have removed the accelerator then setting should be the default. I can't understand why if you have uninstalled, rebooted and installed avast it isn't running on the default settings.
I also can't understand your having removed Netxl when you stated this in the first post "my ISP's mandatory new accelerator." If it is mandatory then without it what happens, that should mean your ISP connection shouldn't work, I'm confused.
Avast has not worked since.
Can you expand this further, error/warning messages, description, etc.
As I said repair won't resolve this.
If you changed any settings based on the information given previously and then removed Netxl then they need to be reversed, but an uninstall, reboot and install of avast should have set things back to the default.
I'm sorry if I'm not grasping the problem I know nothing about Netxl or your ISP's proxy policy or settings, so I'm working in the dark.