I dug up unattended ruby install directions while working on Chef installation directions for windows. Most of the secrets can be found in the RubyInstaller discussion group, such as here and here.

Grab the RubyInstaller for windows, then run: rubyinstaller-1.8.7-p302.exe /tasks="assocfiles,modpath" /silent. The tasks option checks (enables) the options to associated .rb files with ruby and adding the ruby binary directory to the path. You probably wouldn’t want these if you were installing multiple versions of ruby.