title: npm install -g but always installed locally
body: <p>I am using window, and my npm always install my package locally even though I add -g flag after the command: <code>npm install gulp -g</code></p>
  
  <p>How can I solve this problem? I want to share the node module among projects.</p>
  
  <pre><code>&gt;npm -v
  3.10.3
  
  &gt;node -v
  v6.7.0
  
  &gt;npm list -g
  `-- (empty)
  </code></pre>