I wanted to set up an IIS 6.0 application pool for an ASP.NET application and I wanted it to run under a separate local user account. I created the pool and set the correct identity but when I opened the web all I got was a big and ugly "Service Unavailable" message.
In the eventlog I got this message:
Event Type: Warning
Event Source: W3SVC
Event Category: None
Event ID: 1021
Description:
The identity of application pool, 'NewAppPool' is invalid. If it remains invalid when the first request for the application pool is processed, the application pool will be disabled. The data field contains the error number.
It turned out I had forgotten to make the user a member of the IIS Worker Process Group (IIS_WPG) which provides the minimum set of privileges and permissions required to start and run a worker process on a Web server.