Rmagick 2.3 missing - Windows XP

I’ve recently installed the RubyStack 1.4-beta-1 in my Windows XP laptop. Whenever I try to run the server using ‘ruby script/server’, it breaks down and gives me the error:

• Starting Mongrel listening at 0.0.0.0:3000
• Starting Rails with development environment…
  Missing these required gems: rmagick

You’re running: ruby 1.8.6.0 at c:/rubystack/ruby/bin/ruby.exe rubygems 1.2.0 at c:/rubystack/ruby/lib/ruby/gems/1.8

Run `rake gems:install` to install the missing gems.
—-—-—-—-—-—-—-—-—-—-—-—-

But when I run ‘gem.bat list’, it says rmagick 2.3.0 is installed.
Im confused, thanks in advance for your help! :)