What location does Node.js needs to be installed

I am trying to setup a project to work with Grunt. I need to install Node.js for that to run. I have downloaded and installed Node.js in c:\programfiles.

However when I try running npm install in my project c:\Grunt-proj , I get an error 'npm' is not recognized as an internal or external command.

Can someone help me with where to install Node.js? I am trying to follow this tutorial-

http://24ways.org/2013/grunt-is-not-weird-and-hard/

Many thanks,

The location is fine. I also have installed it in Program Files. But if you need to run the command npm from a command prompt then you need to add it to the path of the system. In my case I have C:\Program Files\nodejs