Networking problem Vista-Vista

I have a Mediacenter with Vista Home Premium installad and a Notebook with
Vista Ultimate both connected through a Router the Mediacenter with a cable
and the Notebook wireless.
I have no problem with the Internet access in any of them. I can also see
(in the networking center) the other computer in both of them.
From the Mediacenter I have access to the Notebook (it appears the security
login window and I enter the user and password of one of the users
configured in the Notebook and got no problem) but the problem is when I try
to do the same from the Notebook.
When I do it, the same security login appears, but it does not work with any
of the users I have configured (I have tried both the ones in the Notebook
and the ones in the Mediacenter).
I have 2 users created in the Mediacenter (one administrator and a limited
account, both with a password) and 3 users in the Notebook (one
administrator and two limited accounts, all with a password but one of the
limited accounts that has not).
The only firewall I have installed is the one of Vista which is activated in
both computers.
Also the antivirus is the same in both computers (NOD32).
Thank you in advance for any help you can provide. I have no idea of how to
fix this.
Regards.