まずは、概要をsymfony と Railsの狭間: [Rails 2.0.2]scaffoldをカスタマイズ?でお勉強。を参考に、C:\src\test\lib\generators\scaffold\templates\layout.html.erbを下記のように変更。こっちも参考にしました。
#C:\srcで"rails test"したと思ってください。

<?xml version="1.0" encoding="Shift_JIS"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "">
<html xmlns="" lang="ja" xml:lang="ja">
  <meta http-equiv="Content-Type" content="application/xhtml+xml; charset=Shift_JIS" />
  <meta name="description" content="" />
  <meta name="keywords" content="" />
  <meta name="robots" content="index,follow" />
  <title><%= controller_class_name %>: <%%= controller.action_name %></title>
  <%%= stylesheet_link_tag 'scaffold' %>

<p style="color: green"><%%= flash[:notice] %></p>

<%%= yield %>


Rails 2.1 で Shift-JIS - 涅ir槃naのmoriqさんのコメントを参考にC:\src\test\config\environment.rbを下記のように変更。

# Be sure to restart your server when you modify this file

# Specifies gem version of Rails to use when vendor/rails is not present
RAILS_GEM_VERSION = '2.3.3' unless defined? RAILS_GEM_VERSION

# Bootstrap the Rails environment, frameworks, and default configuration
require File.join(File.dirname(__FILE__), 'boot') do |config|
  # Settings in config/environments/* take precedence over those specified here.
  # Application configuration should go into files in config/initializers
  # -- all .rb files in that directory are automatically loaded.

  # Add additional load paths for your own custom dirs
  # config.load_paths += %W( #{RAILS_ROOT}/extras )

  # Specify gems that this application depends on and have them installed with rake gems:install
  # config.gem "bj"
  # config.gem "hpricot", :version => '0.6', :source => ""
  # config.gem "sqlite3-ruby", :lib => "sqlite3"
  # config.gem "aws-s3", :lib => "aws/s3"

  # Only load the plugins named here, in the order given (default is alphabetical).
  # :all can be used as a placeholder for all plugins not explicitly named
  # config.plugins = [ :exception_notification, :ssl_requirement, :all ]

  # Skip frameworks you're not going to use. To use Rails without a database,
  # you must remove the Active Record framework.
  # config.frameworks -= [ :active_record, :active_resource, :action_mailer ]

  # Activate observers that should always be running
  # config.active_record.observers = :cacher, :garbage_collector, :forum_observer

  # Set default to the specified zone and make Active Record auto-convert to this zone.
  # Run "rake -D time" for a list of tasks for finding time zone names.
  config.time_zone = 'UTC'

  # The default locale is :en and all translations from config/locales/*.rb,yml are auto loaded.
  # config.i18n.load_path += Dir[Rails.root.join('my', 'locales', '*.{rb,yml}')]
  # config.i18n.default_locale = :de

  # Set output encodeing to Shift-JIS.
  # see,
  config.action_controller.default_charset = 'Shift_JIS'