见这里：http://stackoverflow.com/questions/11663590/issues-with-installing-express-js-in-windows-7?rq=1
In later versions of express comand line was migrated to a separate module: express-generetor
说的是最新版本的express，命令行功能已经转移到模块express-generetor，需要单独安装．使用如下命令
npm install -g express-generator
