can only see local machine users when trying to add users allowed for remote desktop access

Hi,

I have a windows 2003 server on my network(its actually a virtual machine
running on windows server 2008), that I have enabled remote access to (via
properties of 'My Pc/Remote')

In the permission of the remote user tab, I can not find any domain users,
if I try to change location it only allows me to see the local machine, and
if i enter manually the domain\user name it always rejects it.

I can see the entire network when I browse the network from file explorer
and I can access shares on remotes machines with no problem, so I am on the
network and authenticated by the domain controller..

How can I get this problem solved ?