The instructions here:
https://secure.phabricator.com/book/phabricator/article/arcanist_windows/
suggest doing this:
arc set-config editor "\"C:\Program Files (x86)\Notepad++\notepad++.exe\" -multiInst -nosession"
But this gives the error:
Usage Exception: Unknown argument 'multiInst'. Try 'arc help'.
A work around is to add the notepad++ directory to the system PATH so you can then do:
arc set-config editor "notepad++.exe -multiInst -nosession"
Which suggests that the problem is something about those quotation marks....are you splitting the string on "?
Windows shell behaviour is a big icky mess, and it doesn't help that there are so many variants of it. T6966, T8821 and T8298 are some of the tickets that describe this.
arc.bat is php -f "%~dp0..\scripts\arcanist.php" -- %*; cmd and php do all their mungling of the command line before arc starts.
If you're running in cygwin or git-bash, escaping is not as bad, and you can probably replace the external " with '.
I don't have php on windows right now, so I can't test things; The internet is suggesting to replace \" with "" or ^".
You can also try arc set-config --trace editor .... to see the string arc is actually getting.