Http 503 Error Service Unavailable in SharePoint 2010

1. Go to Run and Type inetmgr
2. Go to your Application pools.
3. Click on the SharePoint Site with which you are getting the problem.
4.  Click on Advanced Settings and go to Process Model Tab and select Identity.
5. In Application Pool Identity I observe that Custom Account Radio Button is selected.
6. Select Built in Account Radio Button & select the Smart Tag under the Radio Button.
7. In the Smart Tag select Network Service and press ok.
8. Now the problem is solved.