I've been experimenting with NX (see Evaluating NX) and was able to connect to a machine running Windows XP through the NXServer but was prevented from logging in because, the message said, the user account had a "restriction." This seemed strange since the user account was an "administrator" with all the privileges necessary to do just about anything.
Here's the message dialog:
Long story short: the account used to establish a Remote Desktop connection must have a password. Since this computer is well protected behind a firewall and doesn't contain highly sensitive information, the user account did not have a password.
The Remote Desktop dialog clued me in:
