IIS7 under 503 error

Help the teacher to do a static website, want to try how to deploy to the IIS server down, but the installation is correct, when the site is added to the application site after browsing is always error 503 Service unavailable, And after running the site I added after the application pool will automatically stop, feel very depressed! Re-examination a lot of times are so, on the Internet to check some methods, can not, and later in a great God's help, found that the original application pool permissions out of the question

Workaround:

Right-click the app pool program you're using advanced settings--process modules--identify a new built-in account just fine! Try it if you can't.

But I still do not understand, I installed IIS has not moved its built-in account, why this problem? If I try this method, or not, please go to search! For reference only, if someone understand the approximate principle, or have other ideas, please leave a message to tell me, younger brother grateful!!!

IIS7 under 503 error