Weird Behaviour With Logins At Server Level

Hi,


This thread is a reformulation of a prior thread. 

I created a login 'Network service' at server level in Management Studio
express.
I use windows authentification.
Then i defined an user for my database which is associated to login 'Network
service', because the application asp.net uses that account (IIS 6.0). This
user received db_read and db_write roles.

This works.

Now i experimented a little bit and i removed from the logins at server
level the login 'Network service'.
Result: the application still works.
.
Then i removed the Builtinusers login from the login list at server level.
Result: i get the error: "login failed for Network service".

I recreated then the login 'Network Service' at server level but not the
Builtinusers login.
Result: it works again.

My conclusion is: one of the two logins must be in the list: Network Service
or Builtinusers

Is this right?
Why do i get that error when both logins are removed and not only when
Network Service is removed?

Thanks