If you're familiar with vi then gvim should be no problem.  If you're not you can still use gvim in a windows compatible mode.  Either way gvim can be used as a pretty good (not to mention free) IDE. 

First go get gvim at http://www.vim.org/.

Then go get the project "script" at http://www.vim.org/scripts/script.php?script_id=69  If you can't find it try searching for "project"

Now you need to install a nice little ruby gem named "vim-ruby" 

   You can go this with the following steps:

•    gem install vim-ruby --remote
•    vim-ruby-install.rb

   from your command line of choice.

Now one last bit, to make sure you can enable folding which helps a lot with large files remember to do the following step:

To enable folding, download the patched ruby.vim file shown atop this page. Then put the following lines into .vimrc (or ~/.vim/plugin/ruby.vim):

 set foldmethod=syntax
 set foldtext=getline(v:foldstart)
 " you may try out this: set foldcolumn=3
 set fillchars=fold:\ " note the whitespace after  
 " not neccessary but useful (so you can use the TAB key to fold/unfold):
 map <TAB> za

-- originally found at http://www.rubygarden.org/ruby?VimExtensions

You should be on productive dude after this!