pg_autovacuum fails to run as a windows service (beta4)

I am unable to run pg_autovacuum as a windows service. I am using
8.0.0beta4 on Windows XP Pro. When I execute the following to try to
install it as a service:

pg_autovacuum.exe -I -N os_user -W os_pword -U db_user -p 15432 -L my_log

It fails to show up in the services list. The log file shows:

ERROR: The account name is invalid or does not exist, or the password is
invalid for the account name specified.

The OS account name and password ARE correct however. In fact they are
the same OS account name and password that Postgres is already running
under successfully as a windows service on the same machine.

Note that if I instead run pg_autovacuum from the command line, not as a
service, then it works fine:

pg_autovacuum.exe -U db_user -p 15432 -L my_log

Any ideas? Thanks in advance.

Steve McWilliams