<pre>select * from sys.sysusers (nolock) or
select * from sys.database_principals (nolock)</pre>
This will get you the list of “named” users of the database.
<pre>select * from sys.server_principals (nolock)
where type_desc = ‘WINDOWS_LOGIN'</pre>
This will get you the list of users authorized by their Windows authentications. (This is probably what you really want.)