Borodin gave me a clue for this answer. When I went back to the Services GUI to reconfigure the service, I noticed that although I had typed 'Administrator' as the user name, the user name which actually showed up in the panel was '.\Administrator'. Keeping Borodin's comment in mind, it looks like I could specify '.' as the domain.
So...the code which actually worked was:
require 'rubygems'
require 'win32/service'
include Win32
SERVICE_NAME = 'myservice'
# Create a new service
Service.create({
:service_name => SERVICE_NAME,
:service_type => Service::WIN32_OWN_PROCESS,
:description => 'A custom service I wrote just for fun',
:start_type => Service::AUTO_START,
:error_control => Service::ERROR_NORMAL,
:binary_path_name => 'c:\Ruby193\bin\ruby.exe -C c:\ c:\myservice.rb',
:load_order_group => 'Network',
:service_start_name => '.\Administrator',
:password => 'adminpasswd',
:display_name => SERVICE_NAME
})
Service.start SERVICE_NAME