按照網上傳統的方法安裝幾次都沒有成功.最后只有下載gem文件安裝
1.下載ferret-0.11.6-mswin32.gem文件
2.用gem 安裝:gem install ferret-0.11.6-mswin32 --local
3.gem install acts_as_ferret
4.從網上下載ferrect_ext.so文件,copy到 ferret-0.11.5-x86-mswin32\lib目錄（跟ferret.rb同一目錄）
如果直接下載源碼安裝運行ruby setup.rb.會出現"the c extensions could not be installed"的錯誤
手動安裝plugins
將下載的源文件放到vendor/plugins里面,進入該目錄,運行ruby install.rb
出現的問題:
=> Booting WEBrick...
c:/ruby/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/dependencies.rb:442:in `load_missing_constant': uninitialized constant Ferret::Analysis::Analyzer (NameError)
from c:/ruby/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/dependencies.rb:77:in `const_missing'
from E:/rorapp/demo/vendor/plugins/acts_as_ferret/lib/ferret_extensions.rb:17
from c:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
from c:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
from c:/ruby/lib/ruby/gems/1.8/gems/activesupport-2.2.2/lib/active_support/dependencies.rb:155:in `require'
from E:/rorapp/demo/vendor/plugins/acts_as_ferret/lib/acts_as_ferret.rb:28
from c:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
from c:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
... 25 levels...
from c:/ruby/lib/ruby/gems/1.8/gems/rails-2.2.2/lib/commands/server.rb:49
from c:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
from c:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
from script/server:3
解決辦法就是把上面安裝完成ferret后的文件里找到ferrect_ext.so將這個文件放到lib里面.這個是200多K,而從網上下載的才30K.導致上面的問題.