在练习《Node.js入门》文件上传的例子时，遇到以下情况

     throw e; // process.nextTick error, or 'error' event on first tick
          ^
 Error: Cannot find module 'formidable'
     at Function._resolveFilename (module.js:332:11)
     at Function._load (module.js:279:25)
     at Module.require (module.js:354:17)
     at require (module.js:370:17)
     at Object.<anonymous> (e:\code\nodejs\fileupload\server.js:3:18)
     at Module._compile (module.js:441:26)
     at Object..js (module.js:459:10)
     at Module.load (module.js:348:31)
     at Function._load (module.js:308:12)
     at Array.0 (module.js:479:10)

我已经安装了formidable模块

 ├── formidable[@1](/user/1).0.9
 └─┬ socket.io[@0](/user/0).8.7
   ├── policyfile[@0](/user/0).0.4
   ├── redis[@0](/user/0).6.7
   └─┬ socket.io-client[@0](/user/0).8.7
     ├── uglify-js[@1](/user/1).0.6
     ├── websocket-client[@1](/user/1).0.0
     └── xmlhttprequest[@1](/user/1).2.2