Postgres-pr on windows

After installing One-Click Rails and Postgres 8.1 on Windows, I tried to
install the pure-ruby postgres adapter by running

gem install postgres-pr

It installed fine, and I can do this:

irb(main):001:0> require “rubygems”
=> true
irb(main):002:0> require “postgres”
=> true

But then if I specify in my config/database.yml to use

adapter: postgres

and rake migrate, rails says that it can’t find the postgres adapter.

Digging a little deeper and making the database connection manually:

irb(main):002:0> require “dbi”
=> true
irb(main):003:0> DBI::VERSION
=> “0.0.23”
irb(main):005:0> dbh =
DBI.connect(‘dbi:Pg:research_funding_development’, ‘postgres’)
TypeError: is not a class/module
load_driver' from G:/InstantRails/ruby/lib/ruby/site_ruby/1.8/dbi/dbi.rb:401:in_get_full_driver’
G:/InstantRails/ruby/lib/ruby/site_ruby/1.8/dbi/dbi.rb:381:in `connect’
from (irb):5

What gives? Am I doing something wrong here?