Application pool account in MOSS

Starting this new day with assigning a new application pool account to the PKS site.´
Like a lot of things with SharePoint it can be hard to know were to do things, in this case I thought I had to configure the application pool account by hand and that process took me at least 3h.
Anyhow I learned how to get into a hidden SQL database, which is knowledge I probably will never ever use again.

Assigning a new application pool account is done like this:
”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.”

But is it realy that simple in my case on a domain controller I had to give the user access to the ssp user accounts manually. I got this error:

A runtime exception was detected. Details follow.
Message: ‘xxxl’ is not a valid Windows NT name. Give the complete name: <domain\username>

ref:
How to assign a new application pool account.