Download the ferret gem for windows from http://rubyforge.org/frs/?group_id=1028
(E.g) http://rubyforge.org/frs/download.php/34135/ferret-0.11.6-mswin32.gem
Move to the directory where you downloaded the gem (cd Desktop/<download-location>) and do gem install <filename>
(E.g) gem install ferret-0.11.6-mswin32.gem
You may also need,
gem install acts_as_ferret, for your project.