Estou aprendendo React.js e estou usando o Windows 8 OS.i navegue para a minha pasta raiz

1.Created the package.json file by npm init
2. install webpack by npm install -S webpack.now webpack has been downloaded to my modules folder
3. install webpack globally by typing npm install webpack -g
4. i am also having a webpack.config.js in my root folder which contains the source and ouput directory
5. when i type the webpack command i am getting the below error.

webpack não é reconhecido como um comando interno ou externo, programa operável ou arquivo em lote