???
Does anyone know if Avast can be installed and run from USB Drive?
To clean say, your friends laptop before attaching it to your network.
No. It can’t 
Even there isn’t a plugin for Bart PE CD.
You can buy Avast Bart CD (an expensive tool).
Maybe in the future we can see some of its features into the avast Professional version, so do I hope 
The old 16 bit (dos) version can be installed on USB and is free to use.
Other (windows) versions can also be installed on USB, but you have to install a working OS on it first.
You’re right…
;D