I'm going to start of my answer with a question.
Have you ever installed the likes of MS Exchange on an NT server?
If the answer is yes then you know that you need to assign certian rights to the Exchange services to start up every time the server is booted without the need to manually start the sevices.
You are correct about using User Manager For Domains, try this if you want:
Open User Manager For Domains, highlight the user in question, go to Policies then User Rights, you will see at the bottom left of the box a little tick box that says Show Advanced User Rights tick this, you need to give the following permissions for this user so that when the server is booted up (without logging in) his/her account will be started as a service (so to speak)
1. Act as part of the operating system
2. Log on as a service
3. Log on locally
Your user should then be started just like a service every time the server is booted.
Hope this is of use.