Check the following:
1. Hit start run, type mmc and click OK
2. Add ‘Local Users and Groups’ and ‘Local computer Policy’ by clicking Ctrl+M and Add… – PLEASE NOTE: You will find Local computer Policy under Group Policy Object Editor.
3. Navigate to Console Root\Local computer policy\Computer configuration\windows settings\security settings\local policies\user rights assignment and do the following:
3a. Make sure the your user is listed in the ‘Log on as a service’ policy and in the ‘Act as part of the operating system’
4. Navigate to Console root\local users and groups\users. Right click the user and make sure that the user is member of IIS_WPG group.
5. Finally, make absolutely sure that the password of the user in the app pool is correct on the identity tab.
Now, stop the addpool and start it again.