按照网上传统的方法安装几次都没有成功.最后只有下载gem文件安装
1.下载ferret-0.11.6-mswin32.gem文件
2.用gem 安装:gem install ferret-0.11.6-mswin32 --local
3.gem install acts_as_ferret
4.从网上下载ferrect_ext.so文件,copy到 ferret-0.11.5-x86-mswin32\lib目录（跟ferret.rb同一目录）
如果直接下载源码安装运行ruby setup.rb.会出现"the c extensions could not be installed"的错误
手动安装plugins
将下载的源文件放到vendor/plugins里面,进入该目录,运行ruby install.rb
出现的问题:
=> 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
解决办法就是把上面安装完成ferret后的文件里找到ferrect_ext.so将这个文件放到lib里面.这个是200多K,而从网上下载的才30K.导致上面的问题.