OpenStudio comes with the correct version of Ruby for running simulations using the application. The default location which defaults to C:\Program Files\OpenStudio 1.7.5\ruby-install\ruby\ on Windows machines.

The To install an independent version of Ruby installation instructions on Windows use http://rubyinstaller.org/ and then add the OpenStudio website are optional and only required path to use it in the EXE file to your Environment Variables PATH to run RB files from the command prompt. I just followed those instructions and was able to confirm that is works. line.