On several occasions with XP Pro, I've found workstations taking up to 20 minutes to log in to an NT/2000 domain. If this is happening to you, and you run your XP workstation in an NT 4.0 or Win2k mixed mode domain (this will not work with a win2kdomain in native mode), try the following:
Go into your connection properties, and hit the properties button for TCP/IP.
Click the Advanced Button
Click the WINS tab.
In the NetBIOS settings, select Enable NetBIOS over TCP/IP.
Reboot and try to log in again. You should experience very little to no delay now, depending on your connection speed. I've had some good results from this tweak, and I hope it works for you as well.