Windows7上的Ruby-找不到gem

    gem install buildr

    buildr compile

    C:/Ruby192/lib/ruby/site_ruby/1.9.1/rubygems/dependency.rb:247:in `to_specs\': Could not find buildr (>= 0) amongst [atoulme-Antwrap-0.7.1, builder-2.1.2, buildr-1.4.5-x86-mswin32, buildr-as3-0.2.12, diff-lcs-1.1.2,highline-1.5.1, hoe-2.3.3, json_pure-1.4.3, minitar-0.5.3, minitest-1.6.0, net-sftp-2.0.4, net-ssh-2.0.23, rake-0.8.7, rdoc-2.5.8, rjb-1.3.2-x86-mswin32-60, rspec-2.1.0, rspec-core-2.1.0,rspec-expectations-2.1.0, rspec-mocks-2.1.0, rubyforge-2.0.3, rubygems-update-1.8.4, rubyzip-0.9.4, xml-simple-1.0.12] (Gem::LoadError)
    from C:/Ruby192/lib/ruby/site_ruby/1.9.1/rubygems/dependency.rb:256:in `to_spec\'
    from C:/Ruby192/lib/ruby/site_ruby/1.9.1/rubygems.rb:1182:in `gem\'
    from C:/Ruby192/bin/buildr:18:in `<main>\'

   gem list buildr