Windows: where to put pgpass.conf for user "system"

Hi,
I need to let a process running under user "system" call psql to store
some event data into a database.

The process calls a cmd-batchfile with parameters that constructs an
insert-query and fires it off with psql.
So far that works just for real user accounts but not for system.

Where should I put pgpass.conf for user "system"?
AFAIK it hasn't got an %appdata% directory and the one for all users
didn't work.