503 service unavailable IIS OR Application pool stops automatically in IIS7

SharePoint:
 
503 service unavailable:

application pool stops so you start the application pool of the web Application.

I got this error message when my application pool run with user Identity.

.Net:

1.you are given wrong user name or password you got this error message.

Do the following changes in application pool.

1.click on the application pool
2.click on advanced setting on the right pane.
3.click on Identity Under the process model.
4.select built in account under this user network(Ex:network service )
5.select custom account give user name and password