Networking problem

i have a win 2003 server as my domain controller and my workstations are both win xp pro and win 2000 pro. When i browse the network to access my domain controller or other workstations on the network a login window comes up, asking me to log on to the system. This does not happen on the win 2000 pro workstations but only on some of the win xp pro workstations. I also noticed that all these started happening after we migrated to win 2003 server as our domain controller. I have tried to setup the active directory. Please i need help. I don’t want to recieve this login window when i browse my network