I’ve just tried to run TreesizeFree and Avast has just deleted it from my computer, saying it was a virus! It worked fine a couple days ago!
Surely this is a false positive, and a big mistake? :-\

I have since tried to reinstall, and Avast is preventing me. It is also not allowing me to report it as a false positive, or giving me a choice to ignore. This is so annoying >:(

what malware name did avast give?

Win32:Banker-MGC [Trj]

use viruses and worms forum section for reporting False Positives and virus problems

anyway it should be fixed if you run a manuall update https://forum.avast.com/index.php?topic=176583.msg1252146#msg1252146

OK. Thanks.

Updated, and it works now.