This is a post more about embarrassment than value. But it may help some poor soul such as myself. I recently setup a large SharePoint 2007 farm and realized that the main application was running an application pool under the wrong user.
Given my usual developer habits I simply went to IIS, selected the application pool, selected properties, selected the Identity tab and changed the user. I then launched my browser and much to my surprise I got a service unavailable error. Now this is something I was not expecting. Why did my application/web site just die. A little research showed that SharePoint has a slightly more complex model of security and how it works. Any user running the application pool must also be a member of various Groups on the computer.
So as a next step I went to the SharePoint Central Administration (CA) and started to look for any place I can make this change. Since one enters the username and password for the application pool that will run your application while creating an application I kept looking in the application management part of the CA. When I found nothing I did a web search or "googled" for some ideas. I hit upon some SharePoint 2003 posts on the same issue which pointed me to adding my user to various Groups on my machines. But the group names had changed. I used common sense to assign this user to groups that I thought were relevant and tried to launch my application on the browser.
Service unavailable again. Grrrrrrrrrrrr!!!!!!!!!!!
So I gave up. I had to get this working within the hour. So I just deleted the application and created it again. This was a pain for I had to create the application, re-install custom code, re configure stuff etc.
Half way while doing so I happened to be looking at the Central Administration screen under Operations and I saw a link to "Service Accounts." I so hoped this was not what I thought it was. Yes, ladies and gentlemen, it turned out to be exactly what I needed, but a little too late. What should have taken minutes took me an hour. More than the waste of time was the embarrassment of not knowing this was so easy to fix.
So, if you need to change the identity of who runs your application pool just go to Central Administration, Operations, Service Accounts. Select the application, select the application pool, and then change the username and password. Simple.