In IIS 6.0 I created an application pool for a new application and set the identity to a Windows domain account.  When I tried to load the application I was greeted with “Service Unavailable”.  The Windows system event log showed the two messages below:

A failure was encountered while launching the process serving application pool ‘xxx’. The application pool has been disabled.

The identity of application pool ‘xxx’ is invalid, so the World Wide Web Publishing Service can not create a worker process to serve the application pool.  Therefore, the application pool has been disabled.

In order to fix the problem I added the application pool account to the IIS_WPG local group on the web server.