The win32-open3 library provides a working implemetation of the open3 library for MS Windows. In addition, it provides the Open4 class, which also returns pid information. Note that this library is largely unnecessary with Ruby 1.9.x because of its support for native threads.
Gemfile gem 'win32-open3', '~> 0.3.1'
install gem install win32-open3 -v 0.3.1