Here are some commands from my Windows 7 command prompt that show the problem. As you can see, I added everything I could think of to my path, trying to solve the problem. Same DevKit had worked for me before with the same Ruby installation, so I think I installed it right.
E:\ruby\bookmarks3Copy>gem install atomic -v '1.1.9'
ERROR: Error installing atomic:
The 'atomic' native gem requires installed build tools.
Please update your PATH to include build tools or download the DevKit
from 'http://rubyinstaller.org/downloads' and follow the instructions
at 'http://github.com/oneclick/rubyinstaller/wiki/Development-Kit'
E:\ruby\bookmarks3Copy>path
PATH=f:\ruby\DevKit;f:\ruby\DevKit\bin;[snip];f:\ruby\DevKit\bin;f:\programs\git\bin;"F:\Program Files (x86)\KDiff3"
E:\ruby\bookmarks3Copy>where ruby
f:\ruby\Ruby200-x64\bin\ruby.exe
E:\ruby\bookmarks3Copy>gem list atomic
*** LOCAL GEMS ***
atomic (1.1.14)
E:\ruby\bookmarks3Copy>bundle list atomic
DL is deprecated, please use Fiddle
f:/ruby/Ruby200-x64/lib/ruby/gems/2.0.0/gems/atomic-1.1.14