Via InstantRails FAQ, located on http://instantrails.rubyforge.org/wiki/wiki.pl?FAQ
instant rails makes it easy to have multiple rails versions. You can always add /ruby/bin to your path, and start up rails apps as normal too.
Simply adding ruby\bin to the Windows PATH does work, but doesn't provide a full Ruby development environment. It is missing the file associations that invoke Ruby on .rb[w] files, does not have Windows Start Menu entries for the SciTE editor and the FreeRIDE IDE, doesn't have the custom icons for .rb[w] files, and does not create the RUBYOPT=rubygems environment variable needed to make rubygems part of your system.
To rectify this, do the following:
Temporarily rename the directory ...\InstantRails\ruby to something else (like ...\InstantRails\ruby-save). Install the [One-Click Ruby Installer] at ...\InstantRails\ruby. Now that you've got all the nifty system mods, delete the directory ... \InstantRails\ruby and rename ...\InstantRails\ruby-save back to its original name.